In part ii. Spencer recognizes successively likenesses and unlike nesses among phenomena (the effects of the Unknowable), which are segregated into manifestations, vivid (object, non-ego) or faint (subject, ego), and then into space and time, matter and motion and force, of which the last is symbolized by our expe rience of resistance, and is that out of which our ideas of matter and motion are built. Hence the Persistence of Force is the ulti mate basis of knowledge. From it Spencer deduces the indestruc tibility of matter and energy, the equivalence and transformation of forces, the necessity of a rhythm, of Evolution (i.e., integration of matter with concomitant dissipation of motion) and Dissolu tion, and finally the statement of the Law of Evolution as "an integration of matter and concomitant dissipation of motion, dur ing which the matter passes from an indefinite incoherent homo geneity to a definite coherent heterogeneity, and during which the retained motion undergoes a parallel transformation." This process of evolution is due to "the instability of the homogeneous," the "multiplication of effects" and their "segregation," continuing until it ceases in complete "equilibration." Sooner or later, how ever, the reverse process of Dissolution, with its absorption of motion and disintegration of matter must prevail, and these oscillations of the cosmic process will continue without end. It appears, therefore, that Spencer ultimately describes the Knowable in terms of the mechanical conceptions of matter and motion, and this gives a materialistic colouring to his philosophy.
In the Principles of Biology (1864) the chief points are the definition of life as the continuous adjustment of internal to external relations, and the consequent emphasis on the need of adapting the organism to its environment. This does not suffi ciently recognize that the higher organisms largely adjust external to internal relations and adapt their environment to their needs. His universal process of Evolution seems to give Spencer a crite rion of "higher" and "lower" "progression" and "degeneration," independent of the accidents of actual history. The higher (at least in times of "evolution") is the more complex, whether it invariably survives or not. On the other hand, he advances too easily from the maxim that function is prior to structure to the conclusion that the results of use and disuse are immediately incarnate in structural adaptations capable of hereditary trans mission, an inference that has involved him in controversy with Weismann's school.
In his Principles of Psychology (187o-72) Spencer advocates the genetic explanation of the phenomena of the adult human mind by reference to its infant and animal ancestry. On the funda mental question, however, of the psychophysical connection and the derivation of mind from matter, his utterances are neither clear nor consistent. On the one hand, his whole formulation of Evolution in mechanical terms urges him to compose the mind out of homogeneous units of consciousness (or "feeling") "simi lar in nature to those which we know as nervous shocks ; each of which is the correlative of a rhythmical motion of a material unit or group of such units" (§ 62) ; on the other hand, he is ready to amend nervous into psychical shocks, which is no doubt what he ought to have meant but could not say without ruining the illusory bridge between the psychical and the physiological which is suggested in the phrase "nervous shock." And he admits (§ 63) that if we were compelled to choose between translating mental phenomena into physical and its converse, the latter would be preferable. But he finally leaves the relation between the unknow able "substance of Mind" and the unknowable "substance of Mat ter" to the Unknowable. To the theory of knowledge Spencer contributes a "transfigured realism," to mediate between realism and idealism, and the doctrine that "necessary truths," acquired in experience and congenitally transmitted, are a priori to the individual, though a posteriori to the race, to mediate between empiricism and apriorism.
In the Principles of Sociology (1877-96) Spencer's most influ ential ideas have been that of the social organism, of the origina tion of religion out of the worship of ancestral ghosts, of the natural antagonism between nutrition and reproduction, industrial ism and warfare. Politically, Spencer was an individualist of an extreme laissez faire type, and it is in his attitude that the consequences of his pre-Darwinian conception of Evolution are most manifest. But for this, he would hardly have estab lished so absolute an antithesis between industrial and military competition, and would have been readier to see that the law of the struggle for existence, just because it is universal and equally (though differently) operative in every form of society, cannot be appealed to for guidance in deciding between the merits of an industrial or military and of an individualist or socialist society.
In the Principles of Ethics Spencer, though relying on the in trinsic consequences of actions for the guidance of conduct, con ceives the ethical end in a manner intermediate between the hedonist and the evolutionist. The transition from the evolution ist criterion of survival to the criterion of happiness is effected by means of the psychological argument that pleasure promotes function and that living beings must, upon pain of extinction, take pleasure in actions conducive to their survival. Conduct being the adjustment of acts to ends, and good conduct that which is conducive to the preservation of a pleasurable life in a society so adjusted that each attains his happiness without imped ing that of others, life is valuable only if it conduces to happi ness. On the other hand, life must in the long run so conduce because a constant process of adjustment is going on which is bound to lead to a complete adjustment which will be perfect happiness. Spencer concludes that the sense of duty must diminish as moralization increases. In this reasoning Spencer overlooks the possibility of an expansion of the ethical environment. If this is as rapid as the rate of adaptation, there will be no actual growth of adaptation and so no moral progress. Complete adaptation to an infinitely receding ideal is impossible, but Spencer considers that he can both anticipate such a state, and lay down the rules obtaining in it, which will constitute the code of "Absolute Ethics." He conceives it as a state of social harmony so complete that even the antagonism between altruism and egoism will have been over come ; everyone will derive egoistic pleasure from doing such altruistic acts as may still be needed. Originally the socially salu tary action was in the main that which was enjoined on the indi vidual by his political and religious superiors and by social senti ment ; it was also in the main that to which his higher, more com plex and re-representative feelings prompted. Hence the fear with which the political, religious and social controls were regarded came to be associated also with the specifically moral control of lower by higher feelings, and engendered by coercive element in the feeling of obligation. Its authoritativeness depends on the intrinsic salutariness of self-control, and must cease with the resist ance of the lower feelings. Hence Spencer concludes that the sense of duty must diminish as moralization increases. In the preface to the last part of his Ethics (1893) Spencer regrets that "the Doctrine of Evolution has not furnished guidance to the extent he had hoped," but his contributions to ethics are not unlikely to be the most permanently valuable part of his phi losophy.
After completing his system (1896) Spencer continued to revise it, and brought out new editions of the Biology (1898-99) and First Prin ciples (1900). The dates of his chief works are as follows: 1842, Letters to the Nonconformist, "The Proper Sphere of Government"; 1850, Social Statics; 1852, The Theory of Population (cf. part vi. of Biol ogy) ; "The Development Hypothesis" (in Essays, vol. i.) 1853 ; The Universal Postulate (cf. Psychology, part vii.) ; 5854, "the Genesis of Science" ( in Essays, vol. ii.) ; 1855, Principles of Psychology (1 vol.) ; 1857, Progress, its Law and Cause (Essays, vol. i.) 1858, Essays (con taining most of his contributions to the Westminster Review; 1863, vol. ii.; 1885, vol. iii.) ; 1861, Education: Intellectual, Moral, Physical; 1862, First Principles (2nd ed., 1867; 6th, 190o) ; 1864-67, Principles of Biology (2 vols.) ; 1872, Principles of Psychology (2nd ed., in 2 vols.) ; 1873, The Study of Sociology; 1876, vol. i., The Principles of Sociology; vol. ii., Ceremonial Institutions, 1879, Political Institutions, 1882 ; vol. iii., Ecclesiastical Institutions, 1885, completed 1896 ; 1879, The Data of Ethics (part i of Principles of Ethics in 2 VON.; part iv., Justice, 1891; parts ii. and iii., Inductions of Ethics and Ethics of Individual Life, 1892 ; parts v. and vi., Negative and Positive Beneficence, 1893). 1884, Man versus the State. 1886, Factors of Organic Evolution. 1893, Inade quacy of Natural Selection. 1894, A Rejoinder to Professor Weismann and Weismannism once more. 1897, Fragments. 5902, Facts and Com ments. An Autobiography in 2 vols. appeared posthumously in 1904. For a useful summary of his chief doctrines by Spencer himself see his preface to Collins's Epitome of the Synthetic Philosophy; see also: J. A. Thompson, H. Spencer (1906) ; W. H. Hudson, H. Spencer (1908).