The criticisms which have been directed against materialism are all, or nearly all, variations of the principle of heterogeneity. We will not dwell long on this, but simply recollect that, according to this principle, it is impossible to attribute to the brain the capacity of generating consciousness. Physical force can indeed generate physical force under the same or a different form, and it thus produces all the effects which are determined by the laws of nature. But it is impossible to comprehend how physical force can enrich itself at a given moment by a conscious force. Physical force is reduced to movements of bodies and to displacements of atoms; how could a change of position in any inert objects give rise to a judgment, a reasoning, or any phenomenon of the consciousness? It is further said: this idea of function, which materialists here introduce to render more comprehensible the passage from a material body to a spiritual action, contains only an empty explanation, for the function is not essentially distinct by its nature from the organ; it is simply "the organ in activity," it adds to the organ taken in a state of repose but one change, viz. activity, that is to say movement, and, consequently, the function of an organ is material by the same right as the organ. When a muscle contracts, this contraction, which is the proper function of the muscular fibre, consists in a condensation of the muscular protoplasm, and this condensation is a material fact. When a gland enters into activity, a certain quantity of liquid flows into the channels of the gland, and this liquid is caused by a physical and chemical modification of the cellular protoplasm; it is a melting, or a liquefaction, which likewise is material. The function of the nerve cell is to produce movement, or to preserve it, or to direct it; ii is material like the cells. There is therefore nothing in all those functional phenomena which might lead us to understand how a material cause should be capable of engendering a conscious effect. It seems that all materialists have acknowledged that here is the vulnerable point in their theory, for it is the principle of heterogeneity which they have especially combated. But their defence is wanting in frankness, and principally consists in subterfuges. In brief, it affirms that we are surrounded with mystery, that we are not sufficiently learned to have the right to impose limits to the power of matter, and to say to it: "Thou shalt not produce this phenomenon." A materialist theologian declares that he sees no impossibility in stones thinking and arguing, if God, in His infinite power, has decided to unite thought with brute matter. This argument is not really serious; it demands the intervention of so powerful a _Deus ex machina_, that it can be applied equally to all problems; to solve all is to solve none.