Peirce's Theory of Abduction

One task of logic, Peirce held, is to classify arguments so as to determine the validity of each kind. His own classification is interesting because it includes a novel type of argument (called abduction') in addition to the two traditionally recognized types (induction and deduction). It is the purpose of this paper to discuss what Peirce thought to be sufficiently distinctive about abduction to warrant calling it a new kind of argument. But since one finds in his writings on abduction a number of different views it is first necessary to make a few remarks concerning the unity of Peirce's thought. The variation to be found in his views on abduction is, of course, typical of Peirce's writings. It is fashionable today to conclude from the fact that Peirce's writings are fragmentary that his thought was likewise fragmentary. Now it is true that Peirce was frustrated in his ambition to create a unified system. But the lack of unity in his thought has been greatly overemphasized because of a failure to recognize three facts: first, that his logic is foundational to the rest of his philosophy, and hence that his three categories are basic to all his thought; second, that in the scientific spirit Peirce pursued the implications of different hypotheses (and as a consequence varied his terminology from paper to paper); and third, that there was a temporal development in his thought. The last point is particularly important for an understanding of Peirce's varied writings on abduction. Roughly speaking, Peirce's thought falls into two periods.2 In the early period Peirce treated inference, and hence abduction, as an evidencing process. The distinction he drew between induction and abduction corresponds to that between the descriptive, summarizing part of science and the explanatory, theoretical part: induction is an inference from a sample to a whole, while abduction is an inference from a body of data to an explaining hypothesis. In his later period Peirce widened the concept of inference to include methodological processes as well as evidencing processes: induction is the method of testing hypotheses, and abduction includes the method of discovering them.3 This development was a part of Peirce's attempt at system building. During his later period Peirce took the original ideas (pragmatism, synechism, abduction, etc.) discovered in the first period of his thought and attempted to use them as the basis of a grand philosophic system. To this end he worked out a