Both the theories of substance which we have just contrasted are _monistic_ in principle, since the opposition between the two conditions of substance--mass and ether--is not original; moreover, they involve a continuous immediate contact and reciprocal action of the two elements. It is otherwise with the _dualistic_ theories of substance which still obtain in the idealist and spiritualist philosophy, and which have the support of a powerful theology, in so far as theology indulges in such metaphysical speculations. These theories draw a distinction between two entirely different kinds of substance, material and immaterial. Material substance enters into the composition of the bodies which are the object of physics and chemistry; the law of the persistence of matter and force is confined to this world (apart from a belief in its "creation from nothing" and other miracles). Immaterial substance is found in the "spiritual world" to which the law does not extend; in this province the laws of physics and chemistry are either entirely inapplicable or they are subordinated to a "vital force," or a "free will," or a "divine omnipotence," or some other phantom which is beyond the ken of critical science. In truth, these profound errors need no further refutation to-day, for experience has never yet discovered for us a single immaterial substance, a single force which is not dependent on matter, or a single form of energy which is not exerted by material movement, whether it be of mass, or of ether, or of both. Even the most elaborate and most perfect forms of energy that we know--the psychic life of the higher animals, the thought and reason of man--depend on material processes, or changes in the neuroplasm of the ganglionic cells; they are inconceivable apart from such modifications. I have already shown (chap. xi.) that the physiological hypothesis of a special, immaterial "soul-substance" is untenable.