§ [114]. The history of naturalism shows that, as in the case of life, so also in the case of mind, its hypotheses were projected by the Greeks, but precisely formulated and verified only in the modern period of science. In the philosophy of Democritus the soul was itself an atom, finer, rounder, and smoother than the ordinary, but thoroughly a part of the mechanism of nature. The processes of the soul are construed as interactions between the soul and surrounding objects. In sensation, the thing perceived produces images by means of effluxes which impinge upon the soul-atom. These images are not true reports of the outer world, but must be revised by thought before its real atomic structure emerges. For this higher critical exercise of thought Democritus devised no special atomic genesis. The result may be expressed either as the invalidity of such operations of mind as he could provide for in his universe, or the irreducibility to his chosen first principles of the very thought which defined them. Later naturalism has generally sacrificed epistemology to cosmology, and reduced thought to sensation. Similarly, will has been regarded as a highly developed case of instinct. Knowledge and will, construed as sensation and instinct, may thus be interpreted in the naturalistic manner within the field of biology.
Automatism.
§ [115]. But the actual content of sensation, and the actual feelings which attend upon the promptings of instinct, still stubbornly testify to the presence in the universe of something belonging to a wholly different category from matter and motion. The attitude of naturalism in this crucial issue has never been fixed and unwavering, but there has gradually come to predominate a method of denying to the inner life all efficacy and real significance in the cosmos, while admitting its presence on the scene. It is a strange fact of history that Descartes, the French philosopher who prided himself on having rid the soul of all dependence on nature, should have greatly contributed to this method. But it is perhaps not so strange when we consider that every dualism is, after all, symmetrical, and that consequently whatever rids the soul of nature at the same time rids nature of the soul. It was Descartes who first conceived the body and soul to be utterly distinct substances. The corollary to this doctrine was his automatism, applied in his own system to animals other than man, but which those less concerned with religious tradition and less firmly convinced of the soul's originating activity were not slow to apply universally. This theory conceived the vital processes to take place quite regardless of any inner consciousness, or even without its attendance. To this radical theory the French materialists of the eighteenth century were especially attracted. With them the active soul of Descartes, the distinct spiritual entity, disappeared. This latter author had himself admitted a department of the self, which he called the "passions," in which the course and content of mind is determined by bodily conditions. Extending this conception to the whole province of mind, they employed it to demonstrate the thorough-going subordination of mind to body. La Mettrie, a physician and the author of a book entitled "L'Homme Machine," was first interested in this thesis by a fever delirium, and afterward adduced anatomical and pathological data in support of it. The angle from which he views human life is well illustrated in the following:
"What would have sufficed in the case of Julius Cæsar, of Seneca, of Petronius, to turn their fearlessness into timidity or braggartry? An obstruction in the spleen, the liver, or the vena portae. For the imagination is intimately connected with these viscera, and from them arise all the curious phenomena of hypochondria and hysteria. . . . 'A mere nothing, a little fibre, some trifling thing that the most subtle anatomy cannot discover, would have made two idiots out of Erasmus and Fontenelle.'"[250:15]
Radical Materialism. Mind as an Epiphenomenon.
§ [116]. The extreme claim that the soul is a physical organ of the body, identical with the brain, marked the culmination of this militant materialism, so good an instance of that over-simplification and whole-hearted conviction characteristic of the doctrinaire propagandism of France. Locke, the Englishman, had admitted that possibly the substance which thinks is corporeal. In the letters of Voltaire this thought has already found a more positive expression:
"I am body, and I think; more I do not know. Shall I then attribute to an unknown cause what I can so easily attribute to the only fruitful cause I am acquainted with? In fact, where is the man who, without an absurd godlessness, dare assert that it is impossible for the Creator to endow matter with thought and feeling?"[251:16]
Finally, Holbach, the great systematizer of this movement, takes the affair out of the hands of the Creator and definitively announces that "a sensitive soul is nothing but a human brain so constituted that it easily receives the motions communicated to it."[251:17]
This theory has been considerably tempered since the age of Holbach. Naturalism has latterly been less interested in identifying the soul with the body, and more interested in demonstrating its dependence upon specific bodily conditions, after the manner of La Mettrie. The so-called higher faculties, such as thought and will, have been related to central or cortical processes of the nervous system, processes of connection and complication which within the brain itself supplement the impulses and sensations congenitally and externally stimulated. The term "epiphenomenon" has been adopted to express the distinctness but entire dependence of the mind. Man is "a conscious automaton." The real course of nature passes through his nervous system, while consciousness attends upon its functions like a shadow, present but not efficient.[252:18]
Knowledge, Positivism and Agnosticism.