The prevailing conception of the psychic activity, which we contest, considers soul and body to be two distinct entities. These two entities can exist independently of each other; there is no intrinsic necessity for their union. The organized body is a mortal, material nature, chemically composed of living protoplasm and its compounds (plasma-products). The soul, on the other hand, is an immortal, immaterial being, a spiritual agent, whose mysterious activity is entirely incomprehensible to us. This trivial conception is, as such, spiritualistic, and its contradictory is, in a certain sense, materialistic. It is, at the same time, supernatural and transcendental, since it affirms the existence of forces which can exist and operate without a material basis; it rests on the assumption that outside of and beyond nature there is a "spiritual," immaterial world, of which we have no experience, and of which we can learn nothing by natural means. This hypothetical "spirit world," which is supposed to be entirely independent of the material universe, and on the assumption of which the whole artificial structure of the dualistic system is based, is purely a product of poetic imagination; the same must be said of the parallel belief in the "immortality of the soul," the scientific impossibility of which we must prove more fully later on (chap. xi.). If the beliefs which prevail in these credulous circles had a sound foundation, the phenomena they relate to could not be subject to the "law of substance"; moreover, this single exception to the highest law of the cosmos must have appeared very late in the history of the organic world, since it only concerns the "soul" of man and of the higher animals. The dogma of "free will," another essential element of the dualistic psychology, is similarly irreconcilable with the universal law of substance. Our own naturalistic conception of the psychic activity sees in it a group of vital phenomena, which are dependent on a definite material substratum, like all other phenomena. We shall give to this material basis of all psychic activity, without which it is inconceivable, the provisional name of "psychoplasm"; and for this good reason--that chemical analysis proves it to be a body of the group we call protoplasmic bodies the albuminoid carbon-combinations which are at the root of all vital processes. In the higher animals, which have a nervous system and sense-organs, "neuroplasm," the nerve-material, has been differentiated out of psychoplasm. Our conception is, in this sense, materialistic. It is at the same time empirical and naturalistic, for our scientific experience has never yet taught us the existence of forces that can dispense with a material substratum, or of a spiritual world over and above the realm of nature.