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The Cartesian philosophy started from the idea that thought is the ground and proof of existence—“Cogito, ergo sum.” This principle naturally led the school to which it gave rise to regard consciousness as wholly inseparable from mental activity. The same principle passed, through Locke, into the modern English school of metaphysics, and became a fixed idea with nearly all English writers on mental philosophy down to comparatively recent times.
On the Continent, and especially in Germany, another and altogether different course was pursued. Leibniz denied the Cartesian dogma ab initio, and maintained the doctrine of unconscious perception, or latent thought, as a fact which can be verified throughout all the stages of animal life, and in the actual operations of the human mind. From him the idea of unconscious intelligence passed into the principal systems of modern German philosophy, so that the conception of thought being embodied in the various operations of the natural world, and gradually rising higher and higher in the scale of existence until it appears in the form of self-consciousness, is one quite familiar and quite current amongst the German philosophical writers.
More recently, the idea has been revived in this country. It formed, for example, an important element in the lectures of Sir W. Hamilton, and thus gained currency in its purely psychological form. On the side of physiology, the same doctrine was brought forward by Dr. Carpenter, under the title of unconscious cerebration; and was pointed out also, quite independently, by Dr. Laycock, as being an example of the “reflex action of the brain.”