His system is designed to embrace five works; each a distinct treatise, but all closely connected in plan, and treating of the following subjects in the order presented: 1st, First Principles; 2d, Principles of Biology; 3d, Principles of Psychology; 4th, Principles of Sociology; 5th, Principles of Morality. The opening work of the series--_First Principles_--though somewhat of an introductory character, is an independent and completed argument. It consists of two parts: first, "The Unknowable," and second, "The Laws of the Knowable." Unattractive as these titles may seem, they indicate a discussion of great originality and transcendent interest. When public consideration is invited to a system of philosophy so extended as to comprehend the entire scheme of nature and humanity, and so bold as to deal with them in the ripest spirit of science, it is natural that many should ask at the outset how the author stands related to the problem of Religion. Mr. Spencer finds this the preliminary question of his philosophy, and engages with it at the threshold of his undertaking. Before attempting to work out a philosophical scheme, he sees that it is at first necessary to find how far Philosophy can go and where she must stop--the necessary limits of human knowledge, or the circle which bounds all rational and legitimate investigation; and this opens at once the profound and imminent question of the spheres and relation of Religion and Science. Mr. Spencer is a leading representative of that school of thinkers which holds that, as man is finite, he can grasp and know only the finite;--that by the inexorable conditions of thought all real knowledge is relative and phenomenal, and hence that we cannot go behind phenomena to find the ultimate causes and solve the ultimate mystery of being. In such assertions as that "God cannot by any searching be found out;" that "a God understood would be no God at all;" and that "to think God _is_ as we think Him to be is blasphemy," we see the recognition of this idea of the inscrutableness of the Absolute Cause. The doctrine itself is neither new nor limited to a few exceptional thinkers. It is widely affirmed by enlightened science, and pervades nearly all the cultivated theology of the present day. Sir William Hamilton and Dr. Mansel are among its recent and ablest expounders. "With the exception," says Sir William Hamilton, "of a few late absolutist theorizers in Germany, this is perhaps the truth of all others most harmoniously reechoed by every philosopher of every school;" and among these he names Protagoras, Aristotle, St. Augustine, Melanchthon, Scaliger, Bacon, Spinoza, Newton, and Kant.