For of two alternatives one must be true. Either consciousness is the function of a something distinct from the brain, which we call the soul, and a sensation is the mode in which this soul is affected by the motion of a part of the brain; or there is no soul, and a sensation is something generated by the mode of motion of a part of the brain. In the former case, the phenomena of the senses are purely spiritual affections; in the latter, they are something manufactured by the mechanism of the body, and as unlike the causes which set that mechanism in motion, as the sound of a repeater is unlike the pushing of the spring which gives rise to it. The nervous system stands between consciousness and the assumed external world, as an interpreter who can talk with his fingers stands between a hidden speaker and a man who is stone deaf—and Realism is equivalent to a belief on the part of the deaf man, that the speaker must also be talking with his fingers. “Les extrêmes se touchent;” the shibboleth of materialists that “thought is a secretion of the brain,” is the Fichtean doctrine that “the phenomenal universe is the creation of the Ego,” expressed in other language. IV. _The motion of the matter of a sensory nerve may be transmitted through the brain to motor nerves, and thereby give rise to contraction of the muscles to which these motor nerves are distributed; and this reflection of motion from a sensory into a motor nerve may take place without volition, or even contrary to it._ In stating these important truths, Descartes defined that which we now term “reflex action.” Indeed he almost uses the term itself, as he talks of the “animal spirits” as “réfléchis,”[45] from the sensory into the motor nerves. And that this use of the word “reflected” was no mere accident, but that the importance and appropriateness of the idea it suggests was fully understood by Descartes’ contemporaries, is apparent from a passage in Willis’s well-known essay, “De Animâ Brutorum,” published in 1672, in which, in giving an account of Descartes’ views, he speaks of the animal spirits being diverted into motor channels, “velut undulatione reflexâ.”[46] Nothing can be clearer in statement, or in illustration, than the view of reflex action which Descartes gives in the “Passions de l’Âme,” Art. xiii. After recapitulating the manner in which sensory impressions transmitted by the sensory nerves to the brain give rise to sensation, he proceeds:—