I can think of my brain by the analogy which it must have with that of another; but the bridge which leads me from external to internal observations I cannot find; between physical and psychic phenomena there is a gulf which we cannot pass. The soul was regarded by the ancients as a harmony. But how this sublime harmony of imagination, of memory, of the passions, and of thought, results from the vibration of the molecules constituting the brain, no one knows. The road which connects psychic facts with the transformation of energy has not yet been pointed out. I know the chemical transformations which give rise to the mechanical work of the muscles of my hand in writing, but I do not know the processes of my brain which thinks and dictates. Many have thought and asserted, because the muscles and glands of our body grow heated by their work, that the brain and nerves also grew warm during activity. For my part, I doubt the accuracy of the methods used in these experiments, nor shall I be convinced unless it be clearly shown to be a fact. As the nature of the chemical processes taking place in the brain is totally unknown to us, it may be that the brain grows colder during activity. The question can only be decided when we succeed in eliminating the serious complications which the greater flow of blood produces in such cases. Till the present day no one knows what parts of the brain are consumed in order to produce thought; no one can imagine how the molecules of the blood penetrate the mass of cerebral cells and become part of consciousness, and neither do we know how, from the joint life of the single cells, something can arise which represents consciousness and sensitivity. Doctrines are here of no use. When our mind has arrived at the last division of matter, at the last localisation of psychic processes, we feel that it is vain to say we are materialists or spiritualists. All schools are confounded in the nullity of our ignorance. The nature of matter is as incomprehensible as that of spirit. From Lucretius, who gave thirty proofs to demonstrate the materiality of the soul, down to modern materialists, not one step has been taken towards the discovery of the nature of thought. As a matter of fact, many materialists throw down one dogma and build another out of its ruins.