The doctrine has understood this connection as being purely material, and has sought its image in other phenomena which are entirely so. Thus, it has borrowed from physiology the principle of its explanation, it has transported into the domain of thought the idea of function, and it has supposed that the soul is to the body in the relation of function to organ. Intelligence would thus be a cerebral function. To explain intelligence, materialists link it with matter, turn it into a property of matter, and compare it to a movement of matter, and sometimes even to a secretion. So Karl Vogt, the illustrious Genevan naturalist, one day declared, to the great scandal of every one, that the brain secretes the thought as the kidney does urine. This bold comparison seemed shocking, puerile, and false, for a secretion is a material thing while thought is not. Karl Vogt also employed another comparison: the brain produces the thought as the muscle produces movement, and it at once seems less offensive to compare the thought to a movement than to compare it to a liquid secretion. At the present day, an illustration still more vague would be used, such as that of a transformation of energy: chemical energy disengaged by the nerve centres would be thus looked upon as transformed into psychical energy.
However, it matters little what metaphors are applied to for help in explaining the passage from the physical to the mental. What characterises materialist philosophy is its belief in the possibility of such a passage, and its considering it as the genesis of thought. "One calls materialist," says Renouvier, with great exactness, "every philosophy which defines thought as the product of a compound whose elements do not imply thought." A sweeping formula which allows us to foresee all the future avatars of the materialist doctrine, and to class them beforehand in the same category.
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.
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.
In the first place, I reproach materialism with presenting itself as a theory of the generation of the consciousness by the object. We have already reproached idealism with putting itself forward as a theory of the generation of the object by the consciousness. The error of the two systems is produced in a converse direction, but is of the same gravity. The consciousness and its object, we say yet again, constitute the widest division it is possible to effect in the domain of cognition; it is quite as illegitimate to reduce the first term to the second as to reduce the second to the first. To reduce one to the other, by way of affiliation or otherwise, there must first be discovered, then, an identity of nature which does not exist.
In the second place, when one examines closely the explanation materialism has imagined in order to derive thought from an action of matter, it is seen that this representation is rendered completely impossible by all we know of the nature of thought. For the materialist to suppose for one moment that thought is a cerebral function, he must evidently make an illusion for himself as to what thought is, and must juggle with concepts. Perhaps, could we penetrate into his own inmost thought, we should discover that at the moment he supposes a mere cell can manufacture the phenomena of consciousness, some vague image suggests itself to him whereby he identifies these phenomena with a light and subtle principle escaping from the nerve cell, something which resembles an electric effluve, or a will-of-the-wisp, or the flame from a punch-bowl.[46]