The complete list of Spencer's works, pub lished during his lifetime, in the form which they finally assumed, in so far as he regarded them as worthy of permanent place among his productions, is as follows: First, the 'Synthetic Philosophy> itself, consisting of 'First Princi ples' (1 vol.); 'The Principles of Biology' (2 vols.); 'The Principles of Psychology> (2 vols.) ; 'The Principles of Sociology' (3 vols.) ; 'The Principles of Ethics' (2 vols.). Second, 'Essays: Scientific, Political and Speculative,' definitive edition (2 vols.). Third, 'Social Statics,' revised edition (1 vol). Fourth, 'The Study of Sociology' (1 vol.). Fifth, (Educa tion' (1 vol.). Sixth, 'Facts and Comments> (1 vol.). Seventh, 'Various Fragments> (1 vol.). Eighth, 'The Inadequacy of Natural Selection> (1 vol.). Ninth, 'Descriptive Sociology.' The last of these works is, in the main, a collection of material prepared by Spencer's assistants and printed as such, but under Spencer's di rection.
In the most general outlines, what is char acteristic of the philosophy of Spencer may be sketched as follows: In his theory of knowl edge Spencer has many features in common with the classic English empiricism. But this empiricism, in his case, is modified by consid erations due to the doctrine of evolution. Knowledge, namely, as he teaches, arises not merely through the experience of the individ ual, but through the experience of the race so that, in the individual, inherited intellectual tendencies determine the recognition of certain fundamental truths as certain and unquestion able so soon as attention is attracted to the mat ters which these truths express. Thus, mathe matical certainties are based upon a kind of evidence which the individual finds to be abso lute and unquestionable, but which the expe rience of the race has molded into their pres ent forms. In consequence, when we inquire into the foundations of knowledge, we meet with certain assurances, which, in our individ ual cases, are to be tested by what Spencer calls the "universal postulate," namely, "the incon ceivability of the opposite." That is whatever proposition is such that the experiment of try ing to conceive its opposite to be true persist ently fails is to be regarded as a primal neces sity, and is to be accepted as true. If we ask how the human race came to .know this propo sition to be true, the answer is, through the inherited effects of experience. But if we ask why the individual must accept it as true, the answer is, simply because the opposite is incon ceivable. Among the principles thus warranted by the "universal postulate," the principle of the "persistence of force" is prominent. The
fundamental principles of ethics have a similar foundation.
Since the ultimate source of our knowledge lies in the experience of the race, it is impos sible either by individual observation or by means of the inherited effects of experience, to fathom the innermost nature of the real world. We can know only the world as it appears to beings with such an organization as the process of evolution has brought to pass in the case of our race. Any effort to transcend the litn itations thus established leads into the region of what Spencer calls the "Unknowable.) Yet it would be wrong to endeavor to conceive our world as merely a world of phenomena, or of contents of experience, or to accept any form of idealism. For the test of the ity of the opposite" assures us, as one of the absolute certainties, that a world of genuine reality, transcending all our experience, both exists and is unknowable to us. It isequally impossible to deny (or rationally to doubt) the reality of such a world and to attempt any, positive theory as to what its ultimate nature is. Subjective idealism, denying that the un knowable exists at all, is, therefore, according to Spencer, quite unthinkable. But every effort to give a positive account of ultimate reality breaks down by leading us to unavoidable con, tradictions, such as are involved in attempting to conceive the world in its wholeness, as either finite or infinite, as either caused or uncaused, as either absolute or relative, and so forth.
All such epistemological considerations, however, are for Spencer mere preliminaries. His principal philosophical interest lies on the positive side, namely, in the effort to reduce to unity the laws of the °knowable world, that is, the laws which govern the phenomena of nature. Such a unification depends, first, upon recognizing the °persistence of force"— a term by means of which Spencer denotes the con servation of energy, as he conceives that fact. In practice, however, Spencer's term °persist ence of force" is applied in more various ways than those which are customary in case of the usual doctrine of the conservation of energy. The "persistence of force," used as a principle, guarantees us against accepting miracles, against conceiving anything as occurring coun ter to strict natural law, against accepting the doctrine of special creation, and so forth. In brief, the doctrine of the persistence of force gives us a view of what may be called the ana lytic unity of nature, namely, a view of that character which is common to every sort of natural process.