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 in addition to the two traditionally recognized types. 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. (shrink)