C. I. Lewis: History and Philosophy of Logic John Corcoran

C. I. Lewis: History and Philosophy of Logic John Corcoran Transactions of the Charles S. Peirce Society: A Quarterly Journal in American Philosophy, Volume 42, Number 1, Winter 2006, pp. 1-9 (Article) Published by Indiana University Press DOI: https://doi.org/10.1353/csp.2006.0005 For additional information about this article https://muse.jhu.edu/article/200088 [ This content has been declared free to read by the pubisher during the COVID-19 pandemic. ] C. I. Lewis (1883–1964) was the first major figure in history and philosophy of logic—a field that has come to be recognized as a sepa- rate specialty after years of work by Ivor Grat- tan-Guinness and others (Dawson 2003, 257). Lewis was among the earliest to accept the challenges offered by this field; he was the first who had the philosophical and mathematical C. I. Lewis talent, the philosophical, logical, and historical History and Philosophy background, and the patience and dedication of Logic to objectivity needed to excel. He was blessed with many fortunate circumstances, not least John Corcoran of which was entering the field when mathematical logic, after only six decades of toil, had just reaped one of its most important harvests with publication of the monumental Principia Mathematica. It was a time of joyful optimism which demanded an historical account and a sober philosophical critique. Lewis was one of the first to apply to mathematical logic the Aristotelian dictum that we do not understand a living institution until we see it growing from its birth. He will always be regarded as a unique authority on the founding of mathematical logic because he began his meticulous and broadly informed study of its history when it had reached a certain maturity as a field, but before its later explosive development had a chance to cloud our understanding of its founding and formative years. Lewis’s judgment that Boole was the founder of mathematical logic, the person whose work began the continuous development of the subject, stands as a massive obstacle to revisionists whose philosophical or nationalistic commitments render this fact inconvenient. Lewis’s articles and books form an essential part not only of history and philosophy of logic, but of logic itself. His criticism of lapses in rigor in Principia Mathematica served to notify generations of logicians that proof was not to be identified with formalistic manipulation of esoteric formulas. TRANSACTIONS OF THE CHARLES S. PEIRCE SOCIETY Vol. 42, No. 1 ©2006 The welcome and long-awaited publication of Murray Murphey’s mas- terful intellectual biography C. I. Lewis: The Last Great Pragmatist (Albany: SUNY Press, 2005) is occasion to reexamine Lewis’s contribution to this field. Thankfully, Murphey saw fit to include ample discussion of the logical aspect of Lewis’s wide-ranging thought—which in its full scope goes far beyond history and philosophy of logic. As Murphey indicates, logic was a small part of the Lewis legacy. He is regarded as a towering figure by many who have little or no appreciation of his great achievements in history and philosophy of logic. I cannot speculate on whether future philosophers will continue to give high marks to Lewis as an epistemologist, a metaphysician, or a value theorist, but I am confident that no historian of logic will be able TRANSACTIONS to ignore his work as a historian of logic, and I strongly suspect and hope that his stature as a philosopher of logic will only increase. Of course, even had he never written a word of history of logic or of philosophy of logic, as the founder of modern modal logic his status as a logician would have been secure. He has a permanent place in history of logic. This commentary does not begin to do justice to Murphey’s excellent treatment of Lewis’s logic and contribution to history and philosophy of logic. Rather it aims to complement Murphey’s book with comments inter- relating Lewis’s ideas and connecting them with those of others. It is a pleasure to write this commentary, not only as a tribute to Lewis’s contribution to the field but also because I feel a deep affinity with him as a philosopher of logic. I think that Lewis had excellent logical instincts and that he was basically right on important and controversial issues that other “more sophisticated” philosophers of logic were impatient with. Other great philosophers of logic were sometimes dogmatic, cavalier, elitist, arrogant, scientistic, evasive, arbitrary, self-righteous, or dismissive of legitimate criticism; Lewis was humanistic, serious, patient, responsible, sensitive and open-minded. He always gives the impression that he would listen to any informed objection. Many of my views are developments, refinements, or variants of views Lewis held or would have held. I should say at the outset, however, that— contrary to Lewis—I was never attracted to the Frege-Russell logicism that held that all of mathematics is tautological. I could not accept the first step —the idea that number theory is composed exclusively of laws of logic. I could see that ambiguities would make it possible for an intelligent person to construe the equation “5 + 7 = 12” as tautological; but I could not see how such a thing could be said for an inequation such as “5 + 7 ≠ 13”. The view that inequations are tautological is one even Russell himself came to retract, but which Lewis never doubted (Lewis-Langford 1932, 211f, Murphey, 82). Logic Includes Formal Epistemology Before Boole, logic was focused on two central problems of logic as formal epistemology: how to show that a given conclusion follows from given premises that formally imply it, and how to show that a given conclusion does not 2 follow from given premises that do not formally imply it. Aristotle wanted a ofC. I. Lewis: History and Philosophy Logic decisive test or criterion for determining if the conclusion follows, and a decisive test or criterion for determining if the conclusion does not follow. Using other equally traditional terminology, the two central problems were how to establish validity and how to establish invalidity of an arbitrary argument, no matter how many premises or how complicated its propositions. Aristotle did not solve the problem of formal epistemology in its full generality, nor did he claim to—contrary to what a few later authors seemed to have thought (Cohen-Nagel 1934, 110). Although he believed or even knew that he had completed “the logic of categorical propositions”, he never thought that he had completed logic. In the opinion of many logicians today, perhaps not a majority, the full problem has still not been solved (despite occasional enthusiastic statements that it has). Aristotle would never have written what Lewis wrote (1932, 72): “Given premises and conclusion, logic can determine whether this conclusion follows”. This is one of many places where Lewis implicitly aligns himself with Aristotle and Boole by endorsing the traditional view that logic includes for- • mal epistemology, even though at the same time he over-estimates modern Corcoran John achievements. Elsewhere Lewis says that logic can be thought of as an “organon” of demonstrative knowledge or as a “canon of deductive infer- ence” (1932, 235 and Murphey, Ch. 3). As we will see, Lewis does not limit logic to formal epistemology, to the concern with determining validity or invalidity of premise-conclusion arguments. In this regard he agrees with Boole and disagrees with Aristotle. Nevertheless, he never wavers in his con- viction that logic includes formal epistemology at least as a part. In 1957, near the end of his career, he seems to revisit his view of logic as including formal epistemology when he writes: “Logic is concerned only with what is deducible from what . .” (Murphey, 319). Boole’s Thesis: Logic is Mathematical Perhaps the first Boolean Revolution was announced in his 1847 book The Mathematical Analysis of Logic (Cambridge: Macmillan). It was more boldly reit- erated on the last page of his 1848 article “The Calculus of Logic” (Cam- bridge and Dublin Mathematical Journal 3:183–198): The view which these enquiries present of the nature of language . exhibit it not as a mere collection of signs, but as a system of expression, the elements of which are subject to the laws of the thought which they represent. That those laws are as rigorously mathematical as are the laws which govern the purely quantitative conceptions of space and time, of number and magnitude, is a conclusion which I do not hesitate to submit to the exactest scrutiny. With even more confidence he returned to this theme on page 12 of his 1854 masterpiece Laws of Thought: “it is certain that [logic’s] ultimate forms and processes are mathematical”. He again emphasizes his thesis in the last 3 paragraph of the book. Perhaps Boole’s most revolutionary achievement was to persuade the world’s logicians that logical reality is mathematical, or at least that logic should be pursued mathematically. Lewis was one of the first philosophically trained logicians to be fully persuaded of what may be called in retrospect “Boole’s Thesis”: logic is mathematical. This view is a kind of converse of the equally revolutionary “Logistic Thesis” in the form “mathematics is logical”, also accepted by Lewis (Murphey, Ch. 1). Despite the fact that both theses tended to attract superficial or scientistic thinkers who, for example, denigrated the more humanistic or value-oriented side of philosophy, Lewis never lost touch with his deep concern with the human condition, nor was he ever tempted to exag- TRANSACTIONS gerate the importance of logic at the cost of other fields of philosophy (Mur- phey, Chs.

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