The brain is perpetually at work, and it is impossible for the mind to embrace its activities in every part. The greater the attention is in one part, the more vague is the knowledge which we have of contiguous parts, the less vivid are the impressions which the senses transmit from the outside world. We need only recall the well-known example of Archimedes who was killed by a Roman soldier during the siege of Syracuse, while he stood in calm contemplation of some geometrical figures. The whole of our brain is never at work at one time; now it is the one half, then the other which is in action. When looking at the sky or at a wall in a uniform light with only one eye, I found that the field of vision changed alternately from light to dark. This does not depend upon the eye but upon the brain, because unconsciously we use first one eye then the other; and, in the same way, the two hemispheres of the brain do not work simultaneously, sometimes it is the one sometimes the other which is in a state of activity. A French general had lost one half of his brain from a wound which clove the skull. He recovered and retained his intelligence and gaiety, but he used soon to grow tired during conversation and could only continue any intense mental work for a few minutes at a time. There are many philosophers who maintain that a considerable portion of our cerebral activity is purely automatic, so that our mind is often in operation without our being conscious of it. When an idea, says Maudsley,[3] disappears from the horizon of consciousness, it need not vanish totally, but may remain, as it were, latent or veiled, continuing by its movements to awaken, to give rise to, other ideas without our being aware of this activity. But when our consciousness is unexpectedly drawn off from its work, or roused by something which had before occupied it, then we catch the idea at work. [3] Maudsley: _The Physiology of Mind_, p. 305. London, 1876. This opinion is rendered probable by a few phenomena which I observed during my studies of the circulation of the blood in the brain, and we may easily convince ourselves also, if we reflect, how often, quite unexpectedly, names and events occur to us when we were least thinking of them, and which we were unable to recall for a long time and in spite of wearisome efforts, when we wished to do so. And we all know that we are unable to fall asleep at will, so little mastery have we over our thoughts. We direct our minds first to this object then to that, in order to draw it away from that which occupies it and keeps us awake. We try to suppress an idea which torments us by calling other ideas to our assistance in ousting it, and often wait powerless for the coming of that silent oblivion and calm of mind which alone can give us rest.