Removed material from the article:

Scholastic realism

Peirce's at one point describes himself as a "scholastic realist of a somewhat extreme stripe" (CP 5.470). In contrast, some writers call him an idealist, apparently on account of his defining reality as "the object of the final opinion of the scientific community", but this label is based on a peculiar sense of the word idealist and is overall misleading in the case of Peirce, as he consistently and systematically argues that reality is best viewed as independent of mind, at least of minds in particular, if not necessarily of minds in general. The problem of interpretation appears to arise from at least three sources. First, Peirce's use of the word independent needs to be understood in a way that is analogous to its definition in mathematics, where it means orthogonal, or its definition in statistics, where it means uncorrelated. In these senses, independence is a particular kind of relation, not a lack of relation, and certainly not a form of disconnection or exclusion. Second, Peirce did in fact describe himself as being in favor of objective idealism, but what he meant by that is a far cry from ordinary idealism. Third, we need to recognize that scholastic realism is one side of the realist vs. nominalist debate over universals, and not a position in the realist vs. idealist debate about a mind-independent reality. Peirce's scholastic realism in fact supplies essential support for his own thesis of objective idealism regarding the relationship between matter and mind. Two early studies on Peirce’s realism and the influence of Duns Scotus thereon, are the chapter by McKeon in Wiener and Young (1952), and that by Moore in Moore and Robin (1964).

In his first remarks on the realist vs. nominalist debate, Peirce sided with nominalism:

Qualities are fictions; for though it is true that roses are red, yet redness is nothing, but a fiction framed for the purpose of philosophizing; yet harmless so long as we remember that the scholastic realism it implies is false. (CE 1, 307, 1865.)

Here Peirce is explicitly disparaging a position he is well-known for spending most of his life defending. How might we make sense of this apparent contradiction? The temptation is to simply say Peirce changed his mind. After all, since Peirce asserts nominalism in 1865 and scholastic realism in 1868, Peirce may have gone from denying the reality of universals to asserting it. This explanation is most famously given by Max Fisch in his "Peirce’s Progress from Nominalism toward Realism" (1967) and then again in his introduction to Volume 2 of the Chronological Edition of Peirce’s writings (1984). More recently this way of understanding Peirce has been indepenently challenged by Rosa Mayorga in her On Universals (2002) and by Robert Lane in his "Peirce's Early Realism" (2004). Both Mayorga and Lane are troubled by several instances where Peirce’s self assessment of his own intellectual development contradicts Fisch's account of Peirce development. One of these statements appears in 1893 when Peirce states that "never, during the thirty years in which I have been writing on philosophical questions, have I failed in my allegiance to realistic opinions and to certain Scotistic ideas" (CP 6.605). Remarks like these led Lane to conduct a re-evaluation of Peirce’s 1865 declarations for nomianlism, whereupon Lane discovered significant evidence for the same conclusion Mayorga had already reached two years earlier (unbenownst to Lane). Both concluded that the correct way to understand Peirce’s shift from outspoken nominalist to outspoken realist is not by reading into Peirce a change in his fundamental philosophical position, but instead to realize that Peirce merely changed his understanding and use of the terms scholastic realism and nominalism. The reason Peirce calls himself a nominalist in 1865 is because he believes realism to only come in the form offered by Plato:

It has been said that these “abstract names” [blueness, hardness, and loudness] denote qualities and connote nothing. But it seems to me the phrase “denoted object” is nothing but a roundabout expression for a thing…. To say that a quality is denoted is to say it is a thing…. [Such terms] were framed at a time when all men were realists in the scholastic sense and consequently things were meant by them, entities which had no quality but that expressed by the word. They, therefore, must denote these things and connote the qualities they relate to. (Peirce, CE 1, 311–312.)

When Peirce goes on to call universals “fictions,” he is not condemning their truth; he is simply asserting that they do not exist as particulars. This becomes clearer when in the same paper Peirce argues against psychologism in logic, by establishing the same “fictional” status for logic and mathematics that he claims for universals. Now by proving logic fictional, Peirce believes he does logic a favor, that is, by saving it from the psychologists. This suggests that Peirce employed fictional in a rather idiosyncratic way. Many things (including universals) covered by Peirce’s pre-1868 use of fictional came under his post-1868 use of real. Peirce had been using “fictional” to refer to things having no physical existence, and not to imply that something was merely the result of human imagination or fancy. By 1868 at least, Peirce had changed his mind about "reality", holding instead that "fictional" should be contrasted with "independent of what we think about it" (real). He no longer deemed existence as a physical object as a prerequisite for being real, so that a lack of physical existence no longer led Peirce to chatacterize universals as "fictional". That something has blueness can be true independent of what anyone thinks of it, and therefore it can be a part of reality despite the fact blueness never has a physical existence anywhere. Blueness is real independent of what anyone thinks, but it does not exist as an entity because it has no secondness.

Formal perspective

Peirce did not live or work in a vacuum. No one who appreciates his use of phrases like laws of the symbol in their historical context can fail to hear the echoes of George Boole, nor the undertones of the symbolist movement in mathematics inspired by the writings of George Peacock.

At the outset of his Laws of Thought, Boole tells us how he plans to evade the horns of a dilemma that would otherwise threaten to block his inquiry before he can even begin.

In proceeding to these inquiries, it will not be necessary to enter into the discussion of that famous question of the schools, whether Language is to be regarded as an essential instrument of reasoning, or whether, on the other hand, it is possible for us to reason without its aid. I suppose this question to be beside the design of the present treatise, for the following reason, viz., that it is the business of Science to investigate laws; and that, whether we regard signs as the representatives of things and of their relations, or as the representatives of the conceptions and operations of the human intellect, in studying the laws of signs, we are in effect studying the manifested laws of reasoning. (Boole, Laws of Thought, p. 24.)

Boole is saying that the business of science, the investigation of laws, applies itself to the laws of signs at such a level of abstraction that its results are the same no matter whether it finds those laws embodied in objects or in intellects. In short, he does not have to choose one or the other in order to begin. This simple idea is the essence of the formal approach in mathematics, and it is one of the reasons that contemporary mathematicians tend to consider structures that are isomorphic. Peirce uses this depth of perspective for the same reason. It allows him to investigate the forms of triadic sign relations that exist among objects, signs, and interpretants without being blocked by the impossible task of acquiring knowledge of supposedly unknowable things in themselves, whether outward objects or the contents of other minds. Like Aristotle and Boole before him, Peirce replaces these impossible problems with the practical problem of inquiring into the sign relations that exist among commonly accessible objects and publicly accessible signs.

How often do we think of the thing in algebra? When we use the symbol of multiplication we do not even think out the conception of multiplication, we think merely of the laws of that symbol, which coincide with the laws of the conception, and what is more to the purpose, coincide with the laws of multiplication in the object. Now, I ask, how is it that anything can be done with a symbol, without reflecting upon the conception, much less imagining the object that belongs to it? It is simply because the symbol has acquired a nature, which may be described thus, that when it is brought before the mind certain principles of its use — whether reflected on or not — by association immediately regulate the action of the mind; and these may be regarded as laws of the symbol itself which it cannot as a symbol transgress. ("On the Logic of Science" (1865), CE 1, 173.)

The motive themes of the symbolist movement are familiar to anyone who has worked a "story problem" in a mathematics course. One learns to approach the story problem, a roughly realistic representation of a concrete set of circumstances, with the aim of abstracting the appropriate general formula from the mass of concrete details that make up the problem — not all of which data are equally pertinent to the solution and some of which may even be thrown in as distractors. The next step is to derive the logical implications of the abstract formula, generally speaking substituting specific values for some of its variables but just as often leaving other variables unfilled in. The bearing of the formula on the desired answer is obscure at first — that is what makes the problem a problem in the first place. But progressive clarification of the formula leads to an equivalent or implied formula that amounts to an abstract answer or a generic solution to the story problem. Given that, there is nothing more to do but fill in the rest of the concrete data to arrive at the concrete answer or the specific solution to the problem.

The three-phase maneuver for solving a story problem, (1) teasing out, (2) cranking the crank, (3) plugging in, can be articulated in semiotic or sign-relational terms as follows: The first phase passes from the object domain to the sign domain, the second phase passes from the sign domain to the interpretant sign domain, continuing perhaps in a relay of successive passes, and the third phase passes from the last interpretant sign domain back to the object domain.

There are a number of issues that typically arise with the continuing development of a symbolist perspective, in any field of endeavor, over the years of its natural life-cycle. We can see these issues illustrated clearly enough in our story problem paradigm, with its parsing of the problem-solving process into the three phases of abstraction, transformation, and application.

Once the division of labor among the three phases of the process has been in place for a sufficiently long time, each of the three phases will tend to take on a certain degree of independence, sometimes actual and sometimes merely apparent, from the other two phases.

As a side-effect of the increasing independence among the various phases of inquiry, there tend to develop specialized disciplines, each devoted to a single aspect of the initially interactive and integral process. A symptom of this stage of development is that references to the 'independence' of the several phases of inquiry may become confused with or even replaced by assertions of their 'autonomy' from one another.

Returning to the formal sciences of logic and mathematics and focusing on the rise of symbolic logic in particular, all of the above issues were clearly recognized and widely discussed among the movers and shakers of the symbolist movement, with especial mention of George Boole, Augustus De Morgan, Benjamin Peirce, and Charles Peirce.

The first symptoms of a crisis typically arise in connection with questions about the status of the abstract symbols that are 'manipulated' in the transformation phase, to express it in sign-relational terms, the sign-to-sign aspect of semiosis. In the beginning, while it is still evident to everyone concerned that these symbols are mined from the matrix of their usual interpretations, which are generally more diverse than unique, these abstracted symbols are commonly referred to as 'uninterpreted symbols', the sense being that they are transiently detached from their interpretations simply for the sake of extra facility in processing the more general thrust of their meanings, after which intermediary process they will have their concrete meanings restored.

When we start to hear these abstract, general, uninterpreted symbols being described as 'meaningless' symbols, then we can be sure that a certain line in our sand-reckoning has been crossed, and that the crossers thereof have hefted or sublimated 'formalism' to the status of a full-blown Weltanschauung rather than a simple heuristic device.

What we observe here is a familiar form of cyclic process, with the crest of excess followed by the slough of despond. The inflationary boom that raises 'formalism' beyond its formative sphere as one among a host of equally useful heuristic tricks to the status of a totalizing worldview leads perforce to the deflationary bust that makes of 'formalist' a pejorative term.

The point of the foregoing discussion is this, that one of the main difficulties that we have in understanding what the whole complex of words rooted in 'form' meant to Peirce is that we find ourselves, historically speaking, on opposite sides of this cycle of ideas from him.

And so we are required, as so often happens in trying to read a writer of another age, to lift the scales of the years from our eyes, to drop the reticles that have encrusted themselves on our 'reading glasses', our hermeneutic scopes, due to the interpolant philosophical schemata that have managed to enscounce themselves in our unthinking culture over the years that separate us from the writer in question.

Relationships, relations, relatives

The reader of Peirce needs to be aware of the distinction between relations and relatives. Succinctly put, relations are objects and relatives are signs. The term "relative" is short for "relative term", and a relative term is a type of sign that forms the main study of the logic of relatives. A relation, on the other hand, is a type of formal object that is treated in the mathematical theory of relations. There is of course an intimate relationship between the two studies, but like most intimate relationships it has its fair share of intricacies.

The following collection of definitions is practically indispensable.

A relative, then, may be defined as the equivalent of a word or phrase which, either as it is (when I term it a complete relative), or else when the verb "is" is attached to it (and if it wants such attachment, I term it a nominal relative), becomes a sentence with some number of proper names left blank.

A relationship, or fundamentum relationis, is a fact relative to a number of objects, considered apart from those objects, as if, after the statement of the fact, the designations of those objects had been erased.

A relation is a relationship considered as something that may be said to be true of one of the objects, the others being separated from the relationship yet kept in view. Thus, for each relationship there are as many relations as there are blanks. For example, corresponding to the relationship which consists in one thing loving another there are two relations, that of loving and that of being loved by. There is a nominal relative for each of these relations, as "lover of ——" and "loved by ——".

These nominal relatives belonging to one relationship, are in their relation to one another termed correlatives. In the case of a dyad, the two correlatives, and the corresponding relations are said, each to be the converse of the other.

The objects whose designations fill the blanks of a complete relative are called the correlates.

The correlate to which a nominal relative is attributed is called the relate.

In the statement of a relationship, the designations of the correlates ought to be considered as so many logical subjects and the relative itself as the predicate. The entire set of logical subjects may also be considered as a collective subject, of which the statement of the relationship is predicate.

(Peirce, CP 3.466–467, "The Logic of Relatives", Monist, 7, 161–217 (1897), CP 3.456–552).

To understand these definitions, as everywhere in Peirce's work, one needs to keep a close watch on the things that are meant as objects of discussion and thought and the things that are meant as signs and thoughts in which discussion and thought take place. Doing this is trickier than it seems at first, since many standard approaches to defining abstract, formal, or hypostatic objects approach their objects by way of formal operations on the corresponding signs.

Relatives

Relations

A concept of relation that suffices to begin the study of Peirce's logic, mathematics, and semiotics, making use of analogous concepts of relation that have served well enough in other areas of experience to make further experience possibles.

Dyadic relations

Triadic relations

This completes the classification of dual relatives founded on the difference of the fundamental forms A : A and A : B. Similar considerations applied to triple relatives would give rise to a highly complicated development, inasmuch as here we have no less than five fundamental forms of individuals, namely:

(A : A) : A

(A : A) : B

(A : B) : A

(B : A) : A

(A : B) : C.

(Peirce, CP 3.229, "On the Algebra of Logic", American Journal of Mathematics, 3, 15-57 (1880), CP 3.154–251.)

Relations

A concept of relation that suffices to begin the study of Peirce's logic, mathematics, and semiotics, making use of analogous concepts of relation that have served well enough in other areas of experience to make further experience possible, can be set out as follows.

Defined in extension, a k-adic relation L is a set of k-tuples.

A k-tuple x is a sequence of k elements, x₁, x₂, …, x_k–1, x_k, called the components of the k-tuple. The components of the k-tuple x can be indicated by writing either one of the following two forms, the latter form of syntax being the one that Peirce most often used:

x=(x_{1},x_{2},\,\ldots ,x_{j},\ldots ,x_{k-1},x_{k})

x=x_{1}:x_{2}:\cdots :x_{j}:\cdots :x_{k-1}:x_{k}

It is critically important to understand that a relation in extension is a set, in other words, an aggregate entity or a collection of things. More to the point, a single k-tuple is not a relation, it is only an element of a relation, what Peirce quite naturally called an elementary relation or sometimes an individual relation.

In his time, Peirce found himself forced by the task of understanding the intertwined natures of science and signs to develop the logic of relations from the fairly primitive state in which he found it to a condition of readiness more qualified for the job. There was nothing very cut and dried about trying to do this from scratch, as will be evident in the appropriate Sections below when we sample the fits and starts forward, the culs-de-sac, and the many paths that had to be backtracked in order to arrive at an adequate theory of relations. For the purpose at hand, however, we can rely on the fact that few readers these days will have escaped some encounter with relational databases, and so we can draw on these resources of experience to speed the exposition of relations in general.

Table 1 shows how the k-tuples of a k-adic relation might be conceived in tabular form, with the k-uple x_i = (x_i1, …, x_ik) = x_i1 : … : x_ik supplying the entries for the i^th row of the Table.

**Table 1. Relational Database**
Domain 1	Domain 2	...	Domain j	...	Domain k
x₁₁	x₁₂	...	x_1j	...	x_1k
x₂₁	x₂₂	...	x_2j	...	x_2k
...	...	...	...	...	...
x_i1	x_i2	...	x_ij	...	x_ik
...	...	...	...	...	...
x_m1	x_m2	...	x_mJ	...	x_mk

For ease of exposition, Table 1 shows the generic form of a discrete k-adic relation, one that contains a countable number of k-tuples, indeed, it shows a finite k-adic relation, one that contains a finite number of k-tuples. Generalizations to relations with an infinite or even a continuous cardinality in respect of their numbers of elementary relations are possible. Indeed, it is possible to conceive of relations with infinite, continuous, or even no fixed numbers of components in their elementary relations, but finite k-adic relations are illustration enough for our immediate aims.

REMAINDER OF ARTICLE AS OF MARCH 11, 2007, for future reference as needed

Charles Sanders Peirce (pronounced purse), (September 10, 1839 – April 19, 1914) was an American polymath, physicist, and philosopher, born in Cambridge, Massachusetts. Although educated as a chemist and employed as a scientist for 30 years, it is for his contributions to logic, mathematics, philosophy, and the theory of signs, or semeiotic, that he is largely appreciated today. The philosopher Paul Weiss, writing in the Dictionary of American Biography for 1934, called Peirce "the most original and versatile of American philosophers and America's greatest logician" (Brent, 1).

Peirce was largely ignored during his lifetime, and the secondary literature was scant until after World War II. Much of his huge output is still unpublished. Although he wrote mostly in English, he published some popular articles in French as well. An innovator in fields such as mathematics, research methodology, the philosophy of science, epistemology, and metaphysics, he considered himself a logician first and foremost. While he made major contributions to formal logic, "logic" for him encompassed much of what is now called the philosophy of science and epistemology. He, in turn, saw logic as a branch of semiotics, of which he is a founder. In 1886, he saw that logical operations could be carried out by electrical switching circuits, an idea used decades later to produce digital computers.

Life

Brent (1998) is the only Peirce biography in English. Charles Sanders Peirce was the son of Sarah Hunt Mills and Benjamin Peirce, a professor of astronomy and mathematics at Harvard University, perhaps the first serious research mathematician in America. At 12 years of age, Charles read an older brother's copy of Richard Whately's Elements of Logic, then the leading English language text on the subject. Thus began his lifelong fascination with logic and reasoning. He went on to obtain the BA and MA from Harvard, and in 1863 the Lawrence Scientific School awarded him its first M.Sc. in chemistry. This last degree was awarded summa cum laude; otherwise his academic record was undistinguished. At Harvard, he began lifelong friendships with Francis Ellingwood Abbot, Chauncey Wright, and William James. One of his Harvard instructors, Charles William Eliot, formed an unfavorable opinion of Peirce. This opinion proved fateful, because Eliot, while President of Harvard 1869–1909 — a period encompassing nearly all of Peirce's working life — repeatedly vetoed having Harvard employ Peirce in any capacity.

Peirce suffered all his life from what was then known as "facial neuralgia," a very painful nervous/facial condition. The Brent biography says that when in the throes of its pain "he was, at first, almost stupefied, and then aloof, cold, depressed, extremely suspicious, impatient of the slightest crossing, and subject to violent outbursts of temper." His condition would today be diagnosed as trigeminal neuralgia. Its consequences may have led to the social isolation which made the later years of his life so tragic.

United States Coast Survey

Between 1859 and 1891, Charles was intermittently employed in various scientific capacities by the United States Coast Survey, where he enjoyed the protection of his highly influential father until the latter's death in 1880. This employment exempted Charles from having to take part in the Civil War. It would have been very awkward for him to do so, as the Boston Brahmin Peirces sympathized with the Confederacy. At the Survey, he worked mainly in geodesy and in gravimetry, refining the use of pendulums to determine small local variations in the strength of the earth's gravity. The Survey sent him to Europe five times, the first in 1871, as part of a group dispatched to observe a solar eclipse. While in Europe, he sought out Augustus De Morgan, William Stanley Jevons, and William Kingdon Clifford, British mathematicians and logicians whose turn of mind resembled his own. From 1869 to 1872, he was employed as an Assistant in Harvard's astronomical observatory, doing important work on determining the brightness of stars and the shape of the Milky Way. (On Peirce the astronomer, see Lenzen's chapter in Moore and Robin, 1964.) In 1876 he was elected a member of the National Academy of Sciences. In 1878, he was the first to define the meter as so many wavelengths of light of a certain frequency, the definition employed until 1983 (Taylor 2001: 5).

During the 1880s, Peirce's indifference to bureaucratic detail waxed while the quality and timeliness of his Survey work waned. Peirce took years to write reports that he should have completed in mere months. Meanwhile, he wrote hundreds of logic, philosophy, and science entries for the Century Dictionary. In 1885, an investigation by the Allison Commission exonerated Peirce, but led to the dismissal of Superintendent Julius Hilgard and several other Coast Survey employees for misuse of public funds. In 1891, Peirce resigned from the Coast Survey, at the request of Superintendent Thomas Corwin Mendenhall. He never again held regular employment.

Johns Hopkins University

In 1879, Peirce was appointed Lecturer in logic at the new Johns Hopkins University. That university was strong in a number of areas that interested him, such as philosophy (Royce and Dewey did their PhDs at Hopkins), psychology (taught by G. Stanley Hall and studied by Joseph Jastrow, who coauthored a landmark empirical study with Peirce), and mathematics (taught by J. J. Sylvester, who came to admire Peirce's work on mathematics and logic). This untenured position proved to be the only academic appointment Peirce ever held.

Brent documents something Peirce never suspected, namely that his efforts to obtain academic employment, grants, and scientific respectability were repeatedly frustrated by the covert opposition of a major American scientist of the day, Simon Newcomb. Peirce's ability to find academic employment may also have been frustrated by a difficult personality. Brent conjectures that Peirce may have been manic-depressive, claiming that Peirce experienced eight nervous breakdowns between 1876 and 1911. Brent also believes that Peirce tried to alleviate his symptoms with ether, morphine, and cocaine.

Peirce's personal life also proved a grave handicap. His first wife, Harriet Melusina Fay, left him in 1875. He soon took up with a woman whose maiden name and nationality remain uncertain to this day (the best guess is that her name was Juliette Froissy and that she was French), but did not marry her until his divorce with Harriet became final in 1883. That year, Newcomb pointed out to a Johns Hopkins trustee that Peirce, while a Hopkins employee, had lived and traveled with a woman to whom he was not married. The ensuing scandal led to his dismissal. Just why Peirce's later applications for academic employment at Clark University, University of Wisconsin-Madison, University of Michigan, Cornell University, Stanford University, and the University of Chicago were all unsuccessful can no longer be determined. Presumably, his having lived with Juliette for a number of years while still legally married to Harriet led him to be deemed morally unfit for academic employment anywhere in the USA. Peirce had no children by either marriage.

Poverty

In 1887 Peirce spent part of his inheritance from his parents to purchase 2,000 rural acres near Milford, Pennsylvania, land which never yielded an economic return. On that land, he built a large house which he named "Arisbe" and where he spent the rest of his life, writing prolifically, much of it unpublished to this day. His insistence on living beyond his means soon led to grave financial and legal difficulties. Peirce spent much of the last two decades of his life so destitute that he could not afford heat in winter, and his only food was old bread kindly donated by the local baker. Unable to afford new stationery, he wrote on the verso side of old manuscripts. An outstanding warrant for assault and unpaid debts led to his being a fugitive in New York City for a while. Several people, including his brother James Mills Peirce and his neighbors, relatives of Gifford Pinchot, settled his debts and paid his property taxes and mortgage.

Peirce did some scientific and engineering consulting and wrote a good deal for meager pay, primarily dictionary and encyclopedia entries, and reviews for The Nation (with whose editor, Wendell Phillips Garrison he became friendly). He did translations for the Smithsonian Institution, at the instigation of its director, Samuel Langley. Peirce also did substantial mathematical calculations for Langley's research on powered flight. Hoping to make money, Peirce tried his hand at inventing, and began but did not complete a number of books. In 1888, President Grover Cleveland appointed him to the Assay Commission. From 1890 onwards, he had a friend and admirer in Judge Francis C. Russell of Chicago, who introduced Peirce to Paul Carus and Edward Hegeler, the editor and owner, respectively, of the pioneering American philosophy journal The Monist, which eventually published a number of his articles. He applied to the newly formed Carnegie Institution for a grant to write a book summarizing his life's work. The application was doomed; his nemesis Newcomb served on the Institution's executive committee, and its President had been the President of Johns Hopkins at the time of Peirce's dismissal.

The one who did the most to help Peirce in these desperate times was his old friend William James, who dedicated his Will to Believe to Peirce, and who arranged for Peirce to be paid to give four series of lectures at or near Harvard. Most important, each year from 1898 until his death in 1910, James would write to his friends in the Boston intelligentsia, asking that they make a financial contribution to help support Peirce. Peirce reciprocated by designating James's eldest son as his heir should Juliette predecease him, and by adding Santiago, 'Saint James' in Spanish, to his full name (Brent 1998: 315–16, 374).

Peirce died destitute in Milford, Pennsylvania, twenty years before his widow.

Reception

Bertrand Russell opined, "Beyond doubt [...] he was one of the most original minds of the later nineteenth century, and certainly the greatest American thinker ever." (Yet his Principia Mathematica does not mention Peirce.) A. N. Whitehead, while reading some of Peirce's unpublished manuscripts soon after arriving at Harvard in 1924, was struck by how Peirce had anticipated his own "process" thinking. (On Peirce and process metaphysics, see the chapter by Lowe in Moore and Robin, 1964.) Karl Popper viewed Peirce as "one of the greatest philosophers of all times". Nevertheless, Peirce's accomplishments were not immediately recognized. His imposing contemporaries William James and Josiah Royce admired him, and Cassius Jackson Keyser at Columbia and C. K. Ogden wrote about Peirce with respect, but to no immediate effect.

The first scholar to give Peirce his considered professional attention was Royce's student Morris Raphael Cohen, the editor of a 1923 anthology of Peirce's writings titled Chance, Love, and Logic and the author of the first bibliography of Peirce's scattered writings. John Dewey had had Peirce as an instructor at Johns Hopkins, and from 1916 onwards, Dewey's writings repeatedly mention Peirce with deference. His 1938 Logic: The Theory of Inquiry is Peircean through and through. The publication of the first six volumes of the Collected Papers (1931–35), the most important event to date in Peirce studies and one Cohen made possible by raising the needed funds, did not lead to an immediate outpouring of secondary studies. The editors of those volumes, Charles Hartshorne and Paul Weiss, did not become Peirce specialists. Early landmarks of the secondary literature include the monographs by Buchler (1939), Feibleman (1946), and Goudge (1950), the 1941 Ph.D. thesis by Arthur Burks (who went on to edit volumes 7 and 8 of the Collected Papers), and the edited volume Wiener and Young (1952). The Charles S. Peirce Society was founded in 1946. Its Transactions, an academic journal specializing in Peirce, pragmatism, and American philosophy, has appeared since 1965.

In 1949, while doing unrelated archival work, the historian of mathematics Carolyn Eisele (1902–2000) chanced on an autograph letter by Peirce. Thus began her 40 years of research on Peirce the mathematician and scientist, culminating in Eisele (1976, 1979, 1985). Beginning around 1960, the philosopher and historian of ideas Max Fisch (1900–1995) emerged as an authority on Peirce; Fisch (1986) reprints many of the relevant articles, including a wide-ranging survey (Fisch 1986: 422-48) of the impact of Peirce's thought through 1983.

Peirce has come to enjoy a significant international following. There are university research centers devoted to Peirce studies and pragmatism in Brazil, Finland, Germany, and Spain. His writings have been translated into several languages, including German, French, Finnish, and Swedish. Since 1950, there have been French, Italian, and British Peirceans of note. For many years, the North American philosophy department most devoted to Peirce was the University of Toronto's, thanks in good part to the leadership of Thomas Goudge and David Savan. In recent years, American Peirce scholars have clustered at Indiana University - Purdue University Indianapolis, the home of the Peirce Edition Project, and the Pennsylvania State University.

Robert Burch has commented on Peirce's current influence as follows:

Currently, considerable interest is being taken in Peirce's ideas from outside the arena of academic philosophy. The interest comes from industry, business, technology, and the military; and it has resulted in the existence of a number of agencies, institutes, and laboratories in which ongoing research into and development of Peircean concepts is being undertaken. (Burch 2001/2005.)

Works

Peirce's reputation rests largely on a number of academic papers published in American scholarly and scientific journals. These papers, along with a selection of Peirce's previously unpublished work and a smattering of his correspondence, fill the eight volumes of the Collected Papers of Charles Sanders Peirce, published between 1931 and 1958. An important recent sampler of Peirce's philosophical writings is the two volume The Essential Peirce (Houser and Kloesel (eds.) 1992, Peirce Edition Project (eds.) 1998).

The only book Peirce published in his lifetime was Photometric Researches (1878), a monograph on the applications of spectrographic methods to astronomy. While at Johns Hopkins, he edited Studies in Logic (1883), containing chapters by himself and his graduate students. He was a frequent book reviewer and contributor to The Nation, work reprinted in Ketner and Cook (1975–87).

Hardwick (2001) published Peirce's entire correspondence with Victoria, Lady Welby. Peirce's other published correspondence is largely limited to the 14 letters included in volume 8 of the Collected Papers, and the 20-odd pre-1890 items included in the Writings.

Harvard University acquired the papers found in Peirce's study soon after his death, but did not microfilm them until 1964. Only after Richard Robin (1967) catalogued this Nachlass did it become clear that Peirce had left approximately 1650 unpublished manuscripts, totalling 80,000 pages. Eisele (1976, 1985) published some of this work, but most of it remains unpublished. For more on the vicissitudes of Peirce's papers, see (Houser 1989).

The limited coverage, and defective editing and organization, of the Collected Papers led Max Fisch and others in the 1970s to found the Peirce Edition Project, whose mission is to prepare a more complete critical chronological edition, known as the Writings. Only 6 out of a planned 31 volumes have appeared to date, but they cover the period from 1859–1890, when Peirce carried out much of his best-known work.

On a New List of Categories (1867)

Logic of Relatives (1870)

By 1870, the drive that Peirce exhibited to understand the character of knowledge, starting with our partly innate and partly inured models of the world and working up to the conduct of our scientific inquiries into it, having led him to inquire into the three-roled relationship of objects, signs, and impressions of the mind, now brought him to the pass of needing more power in a theory of relations than the available logical formalisms were up to providing. His first concerted effort to supply the gap was rolled out in his paper "Description of a Notation for the Logic of Relatives, Resulting from an Amplification of the Conceptions of Boole's Calculus of Logic". But the nameplate "LOR of 1870" will do for ease of identification.

Logic of Relatives (1883)

Logic of Relatives (1897)

The Simplest Mathematics (1902)

Kaina Stoicheia (1904)

Peirce's philosophy

It is not sufficiently recognized that Peirce’s career was that of a scientist, not a philosopher; and that during his lifetime he was known and valued chiefly as a scientist, only secondarily as a logician, and scarcely at all as a philosopher. Even his work in philosophy and logic will not be understood until this fact becomes a standing premise of Peircian studies. (Max Fisch, in (Moore and Robin 1964, 486).

Peirce was a working scientist for 30 years, and arguably was a professional philosopher only during the five years he lectured at Johns Hopkins. He learned philosophy mainly by reading a few pages of Kant's Critique of Pure Reason, in the original German, every day while a Harvard undergraduate. His writings bear on a wide array of disciplines, including astronomy, metrology, geodesy, mathematics, logic, philosophy, the history and philosophy of science, linguistics, economics, and psychology. This work has become the subject of renewed interest and approval, resulting in a revival inspired not only by his anticipations of recent scientific developments but also by his demonstration of how philosophy can be applied effectively to human problems.

Peirce's writings repeatedly refer to a system of three categories, named Firstness, Secondness, and Thirdness, devised early in his career in reaction to his reading of Aristotle, Kant, and Hegel. He later initiated the philosophical tendency known as pragmatism, a variant of which his life-long friend William James made popular. Peirce believed that any truth is provisional, and that the truth of any proposition cannot be certain but only probable. The name he gave to this state of affairs was "fallibilism". This fallibilism and pragmatism may be seen as playing roles in his work similar to those of skepticism and positivism, respectively, in the work of others.

Pragmatism

Peirce's recipe for pragmatic thinking, going under the label of pragmatism and also known as pragmaticism, is recapitulated in several versions of the so-called pragmatic maxim. Here is one of his more emphatic statements of it:

Consider what effects that might conceivably have practical bearings you conceive the objects of your conception to have. Then, your conception of those effects is the whole of your conception of the object. (CP 5.438.)

William James, among others, regarded two of Peirce's papers, "The Fixation of Belief" (1877) and "How to Make Our Ideas Clear" (1878) as being the origin of pragmatism. Peirce conceived pragmatism to be a method for clarifying the meaning of difficult ideas through the application of the pragmatic maxim. He differed from William James and the early John Dewey, in some of their tangential enthusiasms, in being decidedly more rationalistic and realistic, in several senses of those terms, throughout the preponderance of his own philosophical moods.

Peirce's pragmatism may be understood as a method of sorting out conceptual confusions by linking the meaning of concepts to their operational or practical consequences. This pragmatism bears no resemblance to "vulgar" pragmatism, which misleadingly connotes a ruthless and Machiavellian search for mercenary or political advantage. Rather, Peirce sought an objective method of verification to test the truth of putative knowledge on a way that goes beyond the usual duo of foundational alternatives, namely:

Deduction from self-evident truths, or rationalism;

Induction from experiential phenomena, or empiricism.

His approach is often confused with the latter form of foundationalism, but is distinct from it by virtue of the following three dimensions:

Active process of theory generation, with no prior assurance of truth;

Subsequent application of the contingent theory, aimed toward developing its logical and practical consequences;

Evaluation of the provisional theory's utility for the anticipation of future experience, and that in dual senses of the word: prediction and control. Peirce's appreciation of these three dimensions serves to flesh out a physiognomy of inquiry far more solid than the flatter image of inductive generalization simpliciter, which is merely the relabeling of phenomenological patterns. Peirce's pragmatism was the first time the scientific method was proposed as an epistemology for philosophical questions.

A theory that proves itself more successful in predicting and controlling our world than its rivals is said to be nearer the truth. This is an operational notion of truth employed by scientists. Unlike the other pragmatists, Peirce never explicitly advanced a theory of truth. But his scattered comments about truth have proved influential to several epistemic truth theorists, and as a useful foil for deflationary and correspondence theories of truth.

Pragmatism is regarded as a distinctively American philosophy. As advocated by James, John Dewey, Ferdinand Canning Scott Schiller, George Herbert Mead, and others, it has proved durable and popular. But Peirce did not seize on this fact to enhance his reputation. While it is sometimes stated that James' and other philosophers' use of the word pragmatism so dismayed Peirce that he renamed his own variant pragmaticism, this was not the main reason (Haack, 55). This is revealed by the context in which Peirce introduced the latter term:

But at present, the word [pragmatism] begins to be met with occasionally in the literary journals, where it gets abused in the merciless way that words have to expect when they fall into literary clutches. … So then, the writer, finding his bantling "pragmatism" so promoted, feels that it is time to kiss his child good-by and relinquish it to its higher destiny; while to serve the precise purpose of expressing the original definition, he begs to announce the birth of the word "pragmaticism", which is ugly enough to be safe from kidnappers. (C. S. Peirce, CP 5.414.)

Formal perspective

Logic as formal semiotic

On the Definition of Logic. Logic is formal semiotic. A sign is something, A, which brings something, B, its interpretant sign, determined or created by it, into the same sort of correspondence (or a lower implied sort) with something, C, its object, as that in which itself stands to C. This definition no more involves any reference to human thought than does the definition of a line as the place within which a particle lies during a lapse of time. It is from this definition that I deduce the principles of logic by mathematical reasoning, and by mathematical reasoning that, I aver, will support criticism of Weierstrassian severity, and that is perfectly evident. The word "formal" in the definition is also defined. (Peirce, "Carnegie Application", NEM 4, 54).

In 1902 Peirce applied to the newly established Carnegie Institution for aid "in accomplishing certain scientific work", presenting an "explanation of what work is proposed" plus an "appendix containing a fuller statement". These parts of the letter, along with excerpts from earlier drafts, can be found in NEM 4 (Eisele 1976). The appendix is organized as a "List of Proposed Memoirs on Logic", and No. 12 among the 36 proposals is titled "On the Definition of Logic", the earlier draft of which is quoted in full above.

On Peirce and his contemporaries Ernst Schröder and Frege, Hilary Putnam (1982) wrote:

When I started to trace the later development of logic, the first thing I did was to look at Schröder's Vorlesungen über die Algebra der Logik. This book … has a third volume on the logic of relations (Algebra und Logik der Relative, 1895). [These] three volumes were the best-known logic text in the world among advanced students, and they can safely be taken to represent what any mathematician interested in the study of logic would have had to know, or at least become acquainted with in the 1890s.

While, to my knowledge, no one except Frege ever published a single paper in Frege's notation, many famous logicians adopted Peirce–Schröder notation, and famous results and systems were published in it. Löwenheim stated and proved the Löwenheim–Skolem theorem … in Peirce's notation. In fact, there is no reference in Löwenheim's paper to any logic other than Peirce's. To cite another example, Zermelo presented his axioms for set theory in Peirce–Schröder notation, and not, as one might have expected, in Russell–Whitehead notation.

One can sum up these simple facts (which anyone can quickly verify) as follows: Frege certainly discovered the quantifier first (four years before O. H. Mitchell did so, going by publication dates, which are all we have as far as I know). But Leif Ericson probably discovered America 'first' (forgive me for not counting the native Americans, who of course really discovered it 'first'). If the effective discoverer, from a European point of view, is Christopher Columbus, that is because he discovered it so that it stayed discovered (by Europeans, that is), so that the discovery became known (by Europeans). Frege did 'discover' the quantifier in the sense of having the rightful claim to priority; but Peirce and his students discovered it in the effective sense. The fact is that until Russell appreciated what he had done, Frege was relatively obscure, and it was Peirce who seems to have been known to the entire world logical community. How many of the people who think that 'Frege invented [formal] logic' are aware of these facts?

The main evidence for Putnam's claims is Peirce (1885), published in the premier American mathematical journal of the day. Peano, Ernst Schröder, among others, cited this article. Peirce was apparently ignorant of Frege's work, despite their rival achievements in logic, philosophy of language, and the foundations of mathematics.

Peirce's other major discoveries in formal logic include:

Distinguishing (Peirce, 1885) between first-order and second-order quantification.

Seeing that Boolean calculations could be carried out by means of electrical switches (W5: 421–24), anticipating Claude Shannon by more than 50 years.

Devising the existential graphs, a diagrammatic notation for the predicate calculus. These graphs form the basis of the conceptual graphs of John F. Sowa, and of Sun-Joo Shin's diagrammatic reasoning.

A philosophy of logic, grounded in his categories and semeiotic, can be extracted from Peirce's writings. This philosophy, as well as Peirce's logical work more generally, is exposited and defended in, and in Hilary Putnam (1982), the Introduction to Houser et al (1997), and Dipert's chapter in Misak (2004). Jean Van Heijenoort (1967), Jaakko Hintikka in his chapter in Brunning and Forster (1997), and Brady (2000) divide those who study formal (and natural) languages into two camps: the model-theorists / semanticists, and the proof theorists / universalists. Hintikka and Brady view Peirce as a pioneer model theorist. On how the young Bertrand Russell, especially his Principles of Mathematics and Principia Mathematica, did not do Peirce justice, see Anellis (1995).

Peirce's work on formal logic had admirers other than Ernst Schröder:

The philosophical algebraist William Kingdon Clifford and the logician William Ernest Johnson, both British;

The Polish school of logic and foundational mathematics, including Alfred Tarski;

Arthur Prior, whose Formal Logic and chapter in Moore and Robin (1964) praised and studied Peirce's logical work.

Theory of categories

Now the discovery of ideas as general as these is chiefly the willingness to make a brash or speculative abstraction, in this case supported by the pleasure of purloining words from the philosophers: "Category" from Aristotle and Kant, "Functor" from Carnap (Logische Syntax der Sprache), and "natural transformation" from then current informal parlance. (Saunders Mac Lane, Categories for the Working Mathematician, 29–30.)

Mac Lane did not mention Peirce among the objects of his sincerest flattery, but he might as well have, for his mention of Aristotle and Kant well enough credits his deep indebtedness to the pursers of them all. As Richard Feynman was fond of observing, 'the same questions have the same answers', and the problem that a system of categories is aimed to 'beautify' is the same sort of beast whether it's Aristotle, Kant, Peirce, Carnap, or Eilenberg and Mac Lane that bends the bow. What is that problem? To answer that, let's begin again at the source:

Things are equivocally named, when they have the name only in common, the definition (or statement of essence) corresponding with the name being different. For instance, while a man and a portrait can properly both be called 'animals' (ζωον), these are equivocally named. For they have the name only in common, the definitions (or statements of essence) corresponding with the name being different. For if you are asked to define what the being an animal means in the case of the man and the portrait, you give in either case a definition appropriate to that case alone.
Things are univocally named, when not only they bear the same name but the name means the same in each case -- has the same definition corresponding. Thus a man and an ox are called 'animals'. The name is the same in both cases; so also the statement of essence. For if you are asked what is meant by their both of them being called 'animals', you give that particular name in both cases the same definition. (Aristotle, Categories, 1.1^a1–12.)

In the logic of Aristotle categories are adjuncts to reasoning that are designed to resolve equivocations and thus to prepare ambiguous signs, that are otherwise recalcitrant to being ruled by logic, for the application of logical laws. An equivocation is a variation in meaning, or a manifold of sign senses, and so Peirce's claim that three categories are sufficient amounts to an assertion that all manifolds of meaning can be unified in just three steps.

The following passage is critical to the understanding of Peirce's Categories:

I will now say a few words about what you have called Categories, but for which I prefer the designation Predicaments, and which you have explained as predicates of predicates. That wonderful operation of hypostatic abstraction by which we seem to create entia rationis that are, nevertheless, sometimes real, furnishes us the means of turning predicates from being signs that we think or think through, into being subjects thought of. We thus think of the thought-sign itself, making it the object of another thought-sign. Thereupon, we can repeat the operation of hypostatic abstraction, and from these second intentions derive third intentions. Does this series proceed endlessly? I think not. What then are the characters of its different members? My thoughts on this subject are not yet harvested. I will only say that the subject concerns Logic, but that the divisions so obtained must not be confounded with the different Modes of Being: Actuality, Possibility, Destiny (or Freedom from Destiny). On the contrary, the succession of Predicates of Predicates is different in the different Modes of Being. Meantime, it will be proper that in our system of diagrammatization we should provide for the division, whenever needed, of each of our three Universes of modes of reality into Realms for the different Predicaments. (Peirce, CP 4.549, "Prolegomena to an Apology for Pragmaticism", Monist, 16, 492–546 (1906), CP 4.530–572.)

The first thing that we need to extract from this text is the fact that Categories are predicates of predicates, in effect, types of relations.

Logical graphs

Mathematics

It may be added that algebra was formerly called Cossic, in English, or the Rule of Cos; and the first algebra published in England was called "The Whetstone of Wit", because the author supposed that the word cos was the Latin word so spelled, which means a whetstone. But in fact, cos was derived from the Italian, cosa, thing, the thing you want to find, the unknown quantity whose value is sought. It is the Latin caussa, a thing aimed at, a cause. ("Elements of Mathematics", MS 165 (c. 1895), NEM 2, 50.)

Peirce made a number of striking discoveries in foundational mathematics, nearly all of which came to be appreciated only long after his death. He:

Showed how what is now called Boolean algebra could be expressed by means of a single binary operation, either NAND or its dual, NOR. (See also De Morgan's Laws). This discovery anticipated Sheffer by 33 years.

In Peirce (1885), set out what can be read as the first (primitive) axiomatic set theory, anticipating Zermelo by about two decades.

Discovered the now-classic axiomatization of natural number arithmetic, a few years before Dedekind and Peano did so.

Discovered, independently of Dedekind, an important formal definition of an infinite set, namely, as a set that can be put into a one-to-one correspondence with one of its proper subsets.

Beginning with his first paper on the "Logic of Relatives" (1870), Peirce extended the theory of relations that Augustus De Morgan had just recently woken from its Cinderella slumbers. Much of the actual mathematics of relations that is taken for granted today was "borrowed" from Peirce, not always with all due credit (Anellis 1995). Beginning in 1940, Alfred Tarski and his students rediscovered aspects of Peirce's larger vision of relational logic, developing the perspective of relational algebra. These theoretical resources gradually worked their way into applications, in large part instigated by the work of Edgar F. Codd, who happened to be a doctoral student of the Peirce editor and scholar Arthur W. Burks, on the relational model or the relational paradigm for implementing and using databases.

In the four volume work, The New Elements of Mathematics by Charles S. Peirce (1976), mathematician and Peirce scholar Carolyn Eisele published a large number of Peirce's previously unpublished manuscripts on mathematical subjects, including the drafts for an introductory textbook, allusively titled The New Elements of Mathematics, that presented mathematics from a decidedly novel, if not revolutionary standpoint.

Dynamics of inquiry

Every mind which passes from doubt to belief must have ideas which follow after one another in time. Every mind which reasons must have ideas which not only follow after others but are caused by them. Every mind which is capable of logical criticism of its inferences, must be aware of this determination of its ideas by previous ideas. (Peirce, "On Time and Thought", CE 3, 68-69.)

All through the 1860s, the young but rapidly maturing Charles Peirce was busy establishing a conceptual basecamp and a technical supply line for the intellectual adventures of a lifetime. Taking the longview of this activity and trying to choose the best titles for the story, it all seems to have something to do with the dynamics of inquiry. This broad subject area has a part that is given by nature and a part that is ruled by nurture. On first approach, it is possible to see a question of articulation and a question of explanation:

What is needed to articulate the workings of the active form of representation that is known as conscious experience?

What is needed to account for the workings of the reflective discipline of inquiry that is known as science?

The pursuit of answers to these questions finds them to be so entangled with each other that it's ultimately impossible to comprehend them apart from each other, but for the sake of exposition it's convenient to organize our study of Peirce's assault on the summa by following first the trails of thought that led him to develop a theory of signs, one that has come to be known as 'semiotic', and tracking next the ways of thinking that led him to develop a theory of inquiry, one that would be up to the task of saying 'how science works'.

Opportune points of departure for exploring the dynamics of representation, such as led to Peirce's theories of inference and information, inquiry and signs, are those that he took for his own springboards. Perhaps the most significant influences radiate from points on parallel lines of inquiry in Aristotle's work, points where the intellectual forerunner focused on many of the same issues and even came to strikingly similar conclusions, at least about the best ways to begin. Staying within the bounds of what will give us a more solid basis for understanding Peirce, it serves to consider the following loci in Aristotle:

The basic terminology of psychology, in On the Soul.

The founding description of sign relations, in On Interpretation;

The differentiation of the genus of reasoning into three species of inference that are commonly translated into English as abduction, deduction, and induction, in the Prior Analytics.

In addition to the three elements of inference, that Peirce would assay to be irreducible, Aristotle analyzed several types of compound inference, most importantly the type known as 'reasoning by analogy' or 'reasoning from example', employing for the latter description the Greek word 'paradeigma', from which we get our word 'paradigm'.

Inquiry is a form of reasoning process, in effect, a particular way of conducting thought, and thus it can be said to institute a specialized manner, style, or turn of thinking. Philosophers of the school that is commonly called 'pragmatic' hold that all thought takes place in signs, where 'sign' is the word they use for the broadest conceivable variety of characters, expressions, formulas, messages, signals, texts, and so on up the line, that might be imagined. Even intellectual concepts and mental ideas are held to be a special class of signs, corresponding to internal states of the thinking agent that both issue in and result from the interpretation of external signs.

The subsumption of inquiry within reasoning in general and the inclusion of thinking within the class of sign processes allows us to approach the subject of inquiry from two different perspectives:

The syllogistic approach treats inquiry as a species of logical process, and is limited to those of its aspects that can be related to the most basic laws of inference.

The sign-theoretic approach views inquiry as a genus of semiosis, an activity taking place within the more general setting of sign relations and sign processes.

The distinction between signs denoting and objects denoted is critical to the discussion of Peirce's theory of signs. Wherever needed in the rest of this article, therefore, in order to mark this distinction a little more emphatically than usual, double quotation marks placed around a given sign, for example, a string of zero or more characters, will be used to create a new sign that denotes the given sign as its object.

Theory of signs, or semiotic

Peirce referred to his general study of signs, based on the concept of a triadic sign relation, as semiotic or semeiotic, either of which terms are currently used in either singular of plural form. Peirce began writing on semeiotic in the 1860s, around the time that he devised his system of three categories. He eventually defined semiosis as an "action, or influence, which is, or involves, a cooperation of three subjects, such as a sign, its object, and its interpretant, this tri-relative influence not being in any way resolvable into actions between pairs". (Houser 1998: 411, written 1907). This triadic relation grounds the semeiotic.

In order to understand what a sign is we need to understand what a sign relation is, for signhood is a way of being in relation, not a way of being in itself. In order to understand what a sign relation is we need to understand what a triadic relation is, for the role of a sign is constituted as one among three, where roles in general are distinct even when the things that fill them are not. In order to understand what a triadic relation is we need to understand what a relation is, and here there are traditionally two ways of understanding what a relation is, both of which are necessary if not sufficient to complete understanding, namely, the way of extension and the way of intension. To these traditional approximations, Peirce adds a third way, the way of information, that integrates the other two approaches in a unified whole.

Sign relations

With that hasty map of relations and relatives sketched above, we may now trek into the terrain of sign relations, the main subject matter of Peirce's semeiotic, or theory of signs.

Types of signs

Peirce proposes several typologies and definitions of the signs. More than 76 definitions of what a sign is have been collected throughout Peirce's work. Some canonical typologies can nonetheless be observed, one crucial one being the distinction between "icons", "indices" and "symbols" (CP 2.228, CP 2.229 and CP 5.473). This typology emphasizes the different ways in which the representamen (or its ground) addresses or refers to its object, through a particular mobilisation of an interpretant (but Peirce proposes also other typologies based on other criteria).

An icon is a sign that denotes its objects by virtue of a quality that it shares with them. The sign is perceived as resembling or imitating the object it refers to (e.g. fork on a sign by the road indicating a rest stop). In other words, an icon thus "resembles" to its object. It shares a character or an aspect with it, which allows for it to be interpreted as a sign even if the object does not exist. It signifies essentially on the basis of its "ground".

An index is a sign that denotes its objects by virtue of an existential connection that it has with them. For an index to signify, the relation to the object is crucial. The representamen is directly connected in some way (physically or casually) to the object it denotes (e.g. smoke coming from a building is an index of fire). Hence, an index refers to the object because it is really affected or modified by it, and thus may stand as a trace of the existence of the object.

A symbol is a sign that denotes its objects solely by virtue of the fact that it is interpreted to do so. The representamen does not resemble the object signified but is fundamentally conventional, so that the signifying relationship must be learned and agreed upon (e.g. the word “cat”). A symbol thus denotes, primarily, by virtue of its interpretant. Its action (semeiosis) is ruled by a convention, a more or less systematic set of associations that guarantees its interpretation, independently of any resemblance or any material relation with its object.

Note that these definitions are specific to Peirce's theory of signs and are not exactly equivalent to general uses of the notion of "icon", "symbol" or "index".

Theory of inquiry

Upon this first, and in one sense this sole, rule of reason, that in order to learn you must desire to learn, and in so desiring not be satisfied with what you already incline to think, there follows one corollary which itself deserves to be inscribed upon every wall of the city of philosophy:

Do not block the way of inquiry.

Although it is better to be methodical in our investigations, and to consider the economics of research, yet there is no positive sin against logic in trying any theory which may come into our heads, so long as it is adopted in such a sense as to permit the investigation to go on unimpeded and undiscouraged. On the other hand, to set up a philosophy which barricades the road of further advance toward the truth is the one unpardonable offence in reasoning, as it is also the one to which metaphysicians have in all ages shown themselves the most addicted. (Peirce, "F.R.L." (c. 1899), CP 1.135–136.)

Peirce extracted the pragmatic model or theory of inquiry from its raw materials in classical logic and refined it in parallel with the early development of symbolic logic to address problems about the nature of scientific reasoning. Borrowing a brace of concepts from Aristotle, Peirce examined three fundamental modes of reasoning that play a role in inquiry, processes that are currently known as abductive, deductive, and inductive inference.

In the roughest terms, abduction is what we use to generate a likely hypothesis or an initial diagnosis in response to a phenomenon of interest or a problem of concern, while deduction is used to clarify, to derive, and to explicate the relevant consequences of the selected hypothesis, and induction is used to test the sum of the predictions against the sum of the data.

These three processes typically operate in a cyclic fashion, systematically operating to reduce the uncertainties and the difficulties that initiated the inquiry in question, and in this way, to the extent that inquiry is successful, leading to an increase in the knowledge or skills, in other words, an augmentation in the competence or performance, of the agent or community engaged in the inquiry.

In the pragmatic way of thinking every thing has a purpose, and the purpose of any thing is the first thing that we should try to note about it. The purpose of inquiry is to reduce doubt and lead to a state of belief, which a person in that state will usually call 'knowledge' or 'certainty'. It needs to be appreciated that the three kinds of inference, insofar as they contribute to the end of inquiry, describe a cycle that can be understood only as a whole, and none of the three makes complete sense in isolation from the others.

For instance, the purpose of abduction is to generate guesses of a kind that deduction can explicate and that induction can evaluate. This places a mild but meaningful constraint on the production of hypotheses, since it is not just any wild guess at explanation that submits itself to reason and bows out when defeated in a match with reality. In a similar fashion, each of the other types of inference realizes its purpose only in accord with its proper role in the whole cycle of inquiry. No matter how much it may be necessary to study these processes in abstraction from each other, the integrity of inquiry places strong limitations on the effective modularity of its principal components.

If we then think to inquire, 'What sort of constraint, exactly, does pragmatic thinking place on our guesses?', we have asked the question that is generally recognized as the problem of 'giving a rule to abduction'. Peirce's way of answering it is given in terms of the so-called 'pragmatic maxim', and this in turn gives us a clue as to the central role of abductive reasoning in Peirce's pragmatic philosophy.

Logic of information

Let us now return to the information. The information of a term is the measure of its superfluous comprehension. That is to say that the proper office of the comprehension is to determine the extension of the term. For instance, you and I are men because we possess those attributes — having two legs, being rational, &tc. — which make up the comprehension of man. Every addition to the comprehension of a term lessens its extension up to a certain point, after that further additions increase the information instead. (C.S. Peirce, "The Logic of Science, or, Induction and Hypothesis" (1866), CE 1, 467.)