1. The manifestations of the consciousness are conditioned by the brain. Let us suspend, by any means, the activity of the encephalic mass, by arresting the circulation of the blood for example, and the psychic function is at once inhibited. Compress the carotid, and you obtain the clouding-over of the intellect. Or, instead of a total abolition, you can have one in detail; sever a sensory nerve with the bistoury, and all the sensations which that nerve transmits to the brain are suppressed. Consciousness appears only when the molecular disturbance reaches the nerve centres; everything takes place in the same way as if this disturbance released the consciousness. Consciousness also accompanies or follows certain material states of the nerve centres, such as the waves which traverse the sensory nerves, which exercise reflex action in the cells, and which propagate themselves in the motor nerves. It is to the production, the distribution, and the integrity of this nervous influx that the consciousness is closely linked. It there finds one of the conditions of its apparition. 2. On the other hand, the consciousness remains in complete ignorance of these intra-cerebral phenomena. It does not perceive the nerve-wave which sets it in motion, it knows nothing of its peculiarities, of its trajectory, or the length of its course. In this sense it may be said that it is in no degree an anatomist; it has no idea of all the peculiarities of the nerve-wave which form part of its cerebral history from the moment when these peculiarities are out of relation with the properties of external objects. One sometimes wonders that our consciousness is not aware that the objects we perceive with our two eyes correspond to a double undulation, namely, that of the right and that of the left, and that the image is reversed on the retina, so that it is the rods of the right which are impressed by objects on our left, and the rods of the upper part by objects below our eyes. These are, it has been very justly said, factitious problems, imaginary difficulties which do not exist. There is no need to explain, for instance, direct vision by a reversed image, because our consciousness is not aware that the image on the retina is reversed. In order to take account of this, we should require another eye to see this image. This answer appears particularly to the point. It will be found that it is absolutely correct if we reflect that this case of the unfelt inversion of the image on the retina is but one example of the anatomical ignorance of the consciousness. It might also be declared, in the same order of ideas, that our consciousness is ignorant, that excitements of the eye cross each other at the level of the chiasma, and pass through the internal capsule, and that the majority of the visual excitements of an eye are received by the opposite hemisphere.