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 Notably, the definition of psychical phenomena at which we arrived, not without some trouble, will henceforth play a rather large part in our discussion. It will force us to question a great metaphysical principle which, up till now, has been almost universally considered as governing the problem of the union of the mind with the body.

This principle bears the name of the axiom of heterogeneity, or the principle of psycho-physical dualism. No philosopher has more clearly formulated it, and more logically deduced its consequences, than Flournoy. This author has written a little pamphlet called Métaphysique et Psychologie, wherein he briefly sets forth all the known systems of metaphysics by reducing them to the so-called principle of heterogeneity; after this, the same principle enables him to “execute” them. He formulates it in the following terms: “body and mind, consciousness and the molecular cerebral movement of the brain, the psychical fact and the physical fact, although simultaneous, are heterogeneous, unconnected, irreducible, and obstinately two.” The same author adds: “this is evident of itself, and axiomatic. Every physical, chemical, or physiological event, in the last resort, simply consists, according to science, in a more or less rapid displacement of a certain