That it may well be difficult to explain the nature, differences, causes, and importance of sleep and waking, I think is made clear enough by the fact that concerning them there is great doubt and dissension among the highest philosophers and physicians. For Galen, when he questioned what sleep was, and what waking, decided at length that he could not be certain in what order of phenomena to classify them. Aristotle indeed, in his definitions of sleep and waking, arranges them in different places. Judging from selected books of all authors, no one, in my opinion, has been able to enumerate the general differences of sleep and waking by any certain method. Alcmœn thinks sleep is produced when the blood in the veins flows back and becomes congested. Empedocles believes it to arise from the chilling of heat in the blood. Diogenes states that sleep is induced by the blood pushing to the inner cavities of the body the air that is introduced into bodies. Plato and the Stoics taught that it arose of itself by the letting go of the spirit (or the releasing of the breath) and the consequent relaxation. It is needless to refute the cause which Aristotle gives for sleep, and Galen for waking. They do not accord. The one thinks the true cause arises and has its seat in the heart,—the other in the brain. On account of which disagreement, great contention has been excited among more recent philosophers and physicians, as to which view to adhere to. Some attribute one significance, others another to these things. And so, because of the great difficulty introduced, there is nothing relative to the matter which is not in the deepest obscurity and doubt. A knowledge of this thing is, nevertheless, most useful, even if come upon by any other fortunate means, and not only through the knowledge of the doctors; by such means, for instance, as through the study of the general arts; for if it is joyful to philosophize, and to happen upon the hidden and abstruse things in the secret places of nature,—who would not find great pleasure in learning the causes of the sleeping and waking of creatures, why now they take long, now short sleeps; why at one time it is difficult to capture sleep,—at another time impossible to dispel it? We wonder how at one moment sleep leads to waking; how waking and sleeping mutually succeed each other; why diverse things serve to explain each other, as sleep, waking,—and waking, sleep.