C. I. Lewis

Clarence Irving Lewis (April 12, 1883 – February 3, 1964), usually cited as C. I. Lewis, was an American academic philosopher and the founder of conceptual pragmatism. First a noted logician, he later branched into epistemology, and during the last 20 years of his life, he wrote much on ethics. "The New York Times" memorialized him as "a leading authority on symbolic logic and on the philosophic concepts of knowledge and value."
Biography.
Lewis was born in Stoneham, Massachusetts. His father was a skilled worker in a shoe factory, and Lewis grew up in relatively humble circumstances. He discovered philosophy at age 13, when reading about the Greek pre-Socratics, Anaxagoras and Heraclitus in particular. The first work of philosophy Lewis recalled studying was "A Short History of Greek Philosophy" by John Marshall (1891). Immanuel Kant proved a major lifelong influence on Lewis's thinking. In his article "Logic and Pragmatism", Lewis wrote: "Nothing comparable in importance happened [in my life] until I became acquainted with Kant... Kant compelled me. He had, so I felt, followed scepticism to its inevitable last stage, and laid the foundations where they could not be disturbed."
In 1902, he entered Harvard University. Since his parents were not able to help him financially, he had to work as a waiter to earn his tuition. In 1905, Harvard College awarded Lewis the Bachelor of Arts "cum laude" after a mere three years of study, during which time he supported himself with part-time jobs. He then taught English for one year in a high school in Quincy, Massachusetts, then two years at the University of Colorado. In 1906, he married Mable Maxwell Graves.
In 1908, Lewis returned to Harvard and began a Ph.D. in philosophy, which he completed in a mere two years. He then taught philosophy at the University of California, 1911–20, after which he returned again to Harvard, where he taught until his 1953 retirement, eventually filling the Edgar Pierce Chair of Philosophy. His Harvard course on Kant's first "Critique" was among the most famous in undergraduate philosophy in the U.S. until he retired.
Lewis's life was not free of trials. His daughter died in October 1932 and he suffered a heart attack in 1933. Nevertheless, the publications of Lewis (1929) and Lewis and Langford (1932) attest to these years having been a highly productive period of his life. During this same period, he was elected to the American Academy of Arts and Sciences in 1929, and in 1933, he presided over the American Philosophical Association.
Lewis accepted a visiting professorship at Stanford during 1957–58, where he presented his lectures for the last time. For the academic year 1959–60, he was a Fellow at the Center for Advanced Studies at Wesleyan University.
Philosophical work.
Logic.
Lewis studied logic under his eventual Ph.D. thesis supervisor, Josiah Royce, and is a principal architect of modern philosophical logic. In 1912, two years after the publication of the first volume of "Principia Mathematica", Lewis began publishing articles taking exception to 
"Principia' "s pervasive use of material implication, more specifically, to Bertrand Russell's reading of "a"→"b" as ""a" implies "b"." Lewis restated this criticism in his reviews of both editions of "Principia Mathematica". Lewis's reputation as a promising young logician was soon assured.
Material implication allows a true consequent to follow from a false antecedent. Lewis proposed to replace material implication with strict implication, such that a (contingently) false antecedent does not always strictly imply a (contingently) true consequent. This strict implication was not primitive, but defined in terms of negation, conjunction, and a prefixed unary intensional modal operator, formula_1. If "X" is a formula with a classical bivalent truth value, then formula_1"X" can be read as ""X" is possibly true". Lewis then defined ""A" strictly implies "B"" as "formula_3("A"formula_4"B")". Lewis's strict implication is now a historical curiosity, but the formal modal logic in which he grounded that notion is the ancestor of all modern work on the subject. Lewis' formula_1 notation is still standard, but current practice usually takes its dual, formula_6 ("necessity"), as primitive and formula_1 as defined, in which case ""A" strictly implies "B"" is simply written as formula_6("A"→"B").
His first logic text, "A Survey of Symbolic Logic" (1918), went out of print after selling several hundred copies. At the time of its publication, it included the only discussion in English of the logical writings of Charles Sanders Peirce and only the second monograph, after Russell's of 1900, on Leibniz. While the modal logic of "A Survey" was soon proved inconsistent, Lewis went on to devise the modal systems S1 to S5, and to set these out in "Symbolic Logic" (1932) as possible formal analyses of the alethic modalities. Lewis mildly preferred S2 over the others; the amended modal system of "A Survey" was S3, but it is S4 and S5 that have generated sustained interest, mathematical as well as philosophical, down to the present day. S4 and S5 are the beginning of what is now called normal modal logic. On Lewis' strict implication and his modal systems S1-S5, see Hughes and Cresswell (1996: chapt. 11).
Pragmatist but no positivist.
This section follows Dayton (2004) closely. Around 1930, American philosophy began to experience a turning point because of the arrival of logical empiricism, brought by European philosophers fleeing Nazi Germany. This new doctrine challenged American philosophers of a naturalistic or pragmatic bent, such as Lewis. In any event, logical empiricism, with its emphasis on scientific models of knowledge and on the logical analysis of meaning, soon emerged as a, and perhaps "the", dominant tendency in American philosophy.
While many saw Lewis as kin to the logical empiricists, he was never truly comfortable in such company because he declined to divorce experience from cognition. Positivism rejected value as lacking cognitive significance, also rejecting the analysis of experience in favor of physicalism. Both rejections struck him as regrettable. Indeed, his growing awareness of the pragmatic tradition led him in the opposite direction. For Lewis, it is only within experience that anything can have significance for anything, and thus he came to see value as a way of representing the significance of knowledge for future conduct. These convictions led him to reflect on the differences between pragmatism and positivism, and on the cognitive structure of value experiences.
Lewis agreed that pragmatism committed one to the Peircean pragmatic test. But in a 1930 essay, "Pragmatism and Current Thought," he maintained that this commitment can be taken in either of two directions. One direction emphasises the subjectivity of experience. The other direction, and the one he took in "Mind and the World Order" (1929), began with Peirce's limitation of meaning to that which makes a verifiable difference in experience. Hence concepts are abstractions in which "the immediate is precisely that element which must be left out". But this claim must be properly understood. An operational account of concepts mainly eliminates the ineffable: "If your hours are felt as twice as long as mine, your pounds twice as heavy, that makes no difference, which can be tested, in our assignment of physical properties to things." Hence a concept is but a relational pattern. But it does not follow that one ought to discard the world as it is experienced:
In one sense, that of connotation, a concept strictly comprises nothing but an abstract configuration of relations. In another sense, its denotation or empirical application, this meaning is vested in a process which characteristically begins with something given and ends with something done in the operation which translates a presented datum into an instrument of prediction and control.
Thus knowledge begins and ends in experience, keeping in mind that the beginning and ending experiences differ. Knowledge of something requires that the verifying experience be actually experienced. Thus for the pragmatist, verifiability as an operational definition (or test) of the empirical meaning of a statement requires that the speaker know how to apply the statement, and when not to apply it, and be able to trace the consequences of the statement in situations both real and hypothetical.
Lewis firmly objected to the positivist conception of value statements as devoid of cognitive content, as merely expressive. For a pragmatist, all judgements are implicitly value judgements. Lewis (1946) sets out both his conception of sense meaning, and his thesis that valuation is a form of empirical cognition.
In his essay "Logical Positivism and Pragmatism," Lewis revealed his disagreement with verificationism by comparing it unfavorably with his preferred pragmatic conception of empirical meaning. From the outset, he saw both pragmatism and logical positivism as forms of empiricism. At first glance, it would seem that the pragmatic conception of meaning, despite its different formulation and its focus on action, very much resembles the logical positivist verification requirement. Nevertheless, Lewis argued that there is a deep difference between the two: pragmatism ultimately grounds meaning on conceivable experience, while positivism reduces the relation between meaning and experience to a matter of logical form.
For Lewis, the positivist conception of meaning omits precisely what a pragmatist would count as empirical meaning. Specifying which observation sentences follow from a given sentence helps us determine the empirical meaning of the given sentence only if the observation sentences themselves have an already understood meaning in terms of the specific qualities of experience to which the predicates of the observation sentences refer. Thus Lewis saw the logical positivists as failing to distinguish between "linguistic" meaning, namely the logical relations among terms, and "empirical" meaning, namely the relation expressions have to experience. (In the well-known terminology of Carnap and Charles W. Morris, empirical meaning falls under pragmatics, linguistic meaning under semantics.) For Lewis, the logical positivist shuts his eyes to precisely that which properly confirms a sentence, namely the content of experience.
Epistemology.
Lewis (1929), "Mind and the World Order", is now seen as one of the most important 20th century works in epistemology. Lewis is now included among the American pragmatists, a belated assessment that is the major theme of Murphey (2005).
Lewis was an early exponent of coherentism, particularly as supported by probability observations such as those advocated by Thomas Bayes.
He was the first to employ the term "qualia", popularized by his doctoral student Nelson Goodman, in its generally agreed modern sense.
Ethics and aesthetics.
Lewis's late writings on ethics include the monographs Lewis (1955, 1957) and the posthumous collection Lewis (1969). From 1950 until his death, he wrote many drafts of chapters of a proposed treatise on ethics, which he did not live to complete. These drafts are included in the Lewis papers held at Stanford University.
Lewis (1947) contains two chapters on aesthetics and the philosophy of art.
Legacy.
Lewis's work has been relatively neglected in recent years, even though he set out his ideas at length. He can be understood as both a late pragmatist and an early analytic philosopher, and had students of the calibre of Brand Blanshard, Nelson Goodman, and Roderick Chisholm. Joel Isaac believes this neglect is justified.
Ten lectures and short articles that Lewis produced in the 1950s were collected and edited by John Lange in 1969. The collection, "Values and Imperatives: Studies in Ethics", was published by Stanford University Press.
Lewis's reputation benefits from interest in his contributions to symbolic logic, binary relations, modal logic and the development of pragmatism in American philosophy.
There are 11.5 linear feet of Lewis's papers at Stanford University Libraries.