In 1903 a volume was published by the University of Chicago Press, entitled Studies in Logical Theory, as a part of the "Decennial Publications" of the University. The volume contained contributions by Drs. Thompson (now Mrs. Woolley), McLennan, Ashley, Gore, Heidel, Stuart, and Moore, in addition to four essays by the present writer who was also general editor of the volume. The edition of the Studies being recently exhausted, the Director of the Press suggested that my own essays be reprinted, together with other studies of mine in the same field. The various contributors to the original volume cordially gave assent, and the present volume is the outcome. Chaps. ii-v, inclusive, represent (with editorial revisions, mostly omissions) the essays taken from the old volume. The first and introductory chapter has been especially written for the volume. The other essays are in part reprinted and in part rewritten, with additions, from various contributions to philosophical periodicals. I should like to point out that the essay on "Some Stages of Logical Thought" antedates the essays taken from the volume of Studies, having been published in 1900; the other essays have been written since then. I should also like to point out that the essays in their psychological phases are written from the standpoint of what is now termed a behavioristic psychology, though some of them antedate the use of that term as a descriptive epithet.