[George Trumbull Ladd, Professor of Philosophy, Yale University. b. January 19, 1842, Painesville, Ohio. B.A. Western Reserve College, 1864; B.D. Andover Theological Seminary, 1869; D.D. Western Reserve, 1879; M.A. Yale, 1881; LL.D. Western Reserve, 1895; LL.D. Princeton, 1896. Decorated with the 3d Degree of the Order of the Rising Sun of Japan, 1899; Pastor, Edinburg, Ohio, 1869-71; _ibid._, Milwaukee, Wis., 1871-79; Professor of Philosophy, Bowdoin College, 1879-81; _ibid._, Yale University, 1881--; Lecturer, Harvard, Tokio, Bombay, etc., 1885--, Member American Psychological Association, American Society of Naturalists, American Philosophical Association, American Oriental Society, Imperial Educational Society of Japan, Connecticut Academy. Author of _Elements of Physiological Psychology_; _Philosophy of Knowledge_; _Philosophy of Mind_; _A Theory of Reality_; and many other noted scientific works and papers.] The history of man's critical and reflective thought upon the more ultimate problems of nature and of his own life has, indeed, its period of quickened progress, relative stagnation, and apparent decline. Great thinkers are born and die, "schools of philosophy," so-called, arise, flourish, and become discredited; and tendencies of various characteristics mark the national or more general Zeitgeist of the particular centuries. And always, a certain deep undercurrent, or powerful stream of the rational evolution of humanity, flows silently onward. But these periods of philosophical development do not correspond to those which have been marked off for man by the rhythmic motion of the heavenly bodies, or by himself for purposes of greater convenience in practical affairs. The proposal, therefore, to treat any century of philosophical development as though it could be taken out of, and considered apart from, this constant unfolding of man's rational life is, of necessity, doomed to failure. And, indeed, the nineteenth century is no exception to the general truth.