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.
Modern materialists rightly do not bring God into the question. Their mode of argument takes another form; but it remains to be seen if, at bottom, it is not the same as the other. It simply consists in affirming that up till now we know certain properties of matter only, but that science every day discovers new ones; that matter is a reservoir of unknown forces, and that it is not impossible that the origin of psychical forces may yet be discovered in matter. This idea is clearly hinted at by Littré. The physicist Tyndall gave it a definite formula when he uttered at the Belfast Congress this phrase so often quoted: "If I look back on the limits of experimental science, I can discern in the bosom of that matter (which, in our ignorance, while at the same time professing our respect for its Creator, we have, till now, treated with opprobrium) the promise and the power of all forms and qualities of life."
The opponents of the doctrine have not ceased to answer that the matter of to-morrow, like the matter of to-day, can generate none but material effects, and that a difficulty is not solved by putting off its solution to some indefinite date in our scientific evolution: and it certainly seems that the counter-stroke is decisive, if we admit the principle of heterogeneity with its natural consequence.
We will now criticise the above doctrine by making use of the ideas I have above enunciated. The criticism we have to apply to materialism is not the same as that just summarised. The axis of the discussion changes its position.