---Volume 5 of this edition covers the start of an important transition in Peirce's life. Following his forced resignation from the Johns Hopkins University, Peirce continued to work for the U. S. Coast and Geodetic Survey. His increasing disaffection toward the Survey, however, led to a rekindling of his enthusiasm for speculative philosophy. Volume 5 includes several writings, especially "One, Two, Three," that convey his growing excitement over the synthesizing power of his theory of categories. Many of the writings in Volume 5 are the outcome of work begun by Peirce at Johns Hopkins. Most notable among these are the (unfinished) "Study of Great Men," which Peirce undertook to test the usefulness of statistical methods for comparative biography; the groundbreaking paper in experimental psychology, "On Small Differences of Sensation," written with his student Joseph Jastrow; and the 1885 paper "On the Algebra of Logic," in which Peirce (independently of Frege) introduced quantification theory, the concept of truth-value, and truth-function analysis for testing theorems. Several writings related to this last paper appear here for the first time, and there are other papers for readers interested in the evolution of Peirce's logic and the general history of logic. Also included in Volume 5 are scientific papers that Peirce contributed to the annual reports of the Coast Survey and an account of his testimony to Congress about the organization of the Survey. Perhaps most interesting for the philosopher is Peirce's review of Royce's Religious Aspect of Philosophy and the various papers in which Peirce developed his all-embracing theory of categories toward his 1887-1888 "Guess at the Riddle."