First of all, this is obvious, that waking and sleep belong to the same part of the animal. For they are opposites and sleep appears to be a kind of privation of sleep. For opposites always, both in natural cases and otherwise, happen in the same receptive part, and are affections of the same thing. I mean, for example, health and sickness, beauty and homeliness, strength and weakness, seeing and blindness, and hearing and deafness. And yet also it is obvious from these. For by whatever means we distinguish an awake man, by this same means we distinguish a sleeping man. For we consider a a man who is perceiving to be awake, and every one who is awake either perceives something outside himself or motions in himself. If therefore, waking is in nothing other than perceiving, it is obvious that by the same means there is perception, by this same means both waking things are awake and sleeping things sleep. On Sleep and Waking 453b25-454a7

On Sleep and Waking is a small treatise by Aristotle. The starting point of the philosopher’s inquiry is, as good Aristotelian precedent would often tell us, to begin with what is obvious. Waking and sleep are opposites, because sleep is a lack of waking. Presumably, waking could also be described as a lack of sleep, as every insomniac well knows. In general, that is, in every case, opposites occur in the same faculty, with Aristotle providing examples of health and sickness (the faculty of the body) and hearing and deafness (the faculty of the ear). Since we already know that sleeping and waking are opposites, we have merely to determine in which faculty or place they share their common origin. The perceptive ability is this shared “location” when it comes to waking and sleeping. Waking then, is the use or disuse of the perceptive faculty. We will have to wait, when it comes to defining dreams (in his On Dreams), how Aristotle is able to come up with a definition which avoids overlap between the meaning of waking which I just gave, and dreaming.