The history of philosophical investigation yields no one universally received conception of what philosophy is, nor would the definition given above be unreservedly accepted. Windelband, in his _History of Philosophy_(8) instances the following predominant conceptions of philosophy according to the chronological order in which they prevailed: (_a_) the systematic investigation of the problems raised by man and the universe (early Grecian philosophy: absence of differentiation of philosophy from the special sciences); (_b_) the practical art of human conduct, based on rational speculation (later Grecian philosophy: distrust in the value of knowledge, and emphasis on practical guidance of conduct); (_c_) the helper and handmaid of the Science of Revealed Truth, _i.e._ supernatural theology, in the solution of ultimate problems (the Christian philosophy of the Fathers of the Church and of the Medieval Schools down to the sixteenth century: universal recognition of the value of the Christian Revelation as an aid to rational investigation); (_d_) a purely rational investigation of those problems, going beyond the investigations of the special sciences, and either abstracting from, or denying the value of, any light or aid from Revelation (differentiation of the domains of science, philosophy and theology; modern philosophies from the sixteenth to the nineteenth century; excessive individualism and rationalism of these as unnaturally divorced from recognition of, and belief in, Divine Revelation, and unduly isolated from the progressing positive sciences); (_e_) a critical analysis of the significance and scope and limitations of human knowledge itself (recent philosophies, mainly concerned with theories of knowledge and speculations on the nature of the cognitive process and the reliability of its products). These various conceptions are interesting and suggestive; much might be said about them, but not to any useful purpose in a brief introductory chapter. Let us rather, adopting the definition already set forth, try next to map out into its leading departments the whole philosophical domain.