'The imaginations of them that sleep are those we call dreams; and these also (as all other imaginations) have been before, either totally or by parcels, in the sense. And because in sense the brain and nerves, which are the necessary organs of sense, are so benumbed in sleep as not easily to be moved by the action of external objects, there can happen no sleep, no imagination, and therefore no dream, but what proceeds from the agitation of the inward parts of a man's body; which inward parts, for the connexion they have with the brain and other organs, when they be distempered, do keep the same in motion, whereby the imaginations there formerly made appear as if a man were waking. Saving that the organs of sense being now benumbed so as there is no new object which can master and obscure them with a more vigorous impression, a dream must needs be more clear, in this silence of sense, than are our waking thoughts; and hence it cometh to pass that it is a hard matter, and by many thought impossible, to distinguish exactly between sense and dreaming. For my part, when I consider that in dreams I do not often, nor constantly think of the same persons, places, objects, and actions that I do waking, nor re-mombor so long a train of coherent thoughts dreaming as at other times, and because waking I often observe the absurdity of dreams, but never dream of the absurdities of my waking thoughts, I am well satisfied that being awake I know I dream not, though when I dream I think myself awake.

'And seeing dreams are caused by the distemper of some of the inward parts of the body, divers distempers must cause different dreams. And hence it is that lying cold breedeth dreams of fear, and raiseth the thought and image of some fearful object (the motion from the brain to the inner parts, and from the inner parts to the brain, being reciprocal); and that, as anger causeth heat in some parts of the body when we are awake, so when we sleep the overheating of the same parts causeth anger, and raiseth up in the brain the imagination of an enemy. In the same manner, as natural kindness, when we are awake, causeth desire, and desire makes heat in certain other parts of the body, so also too much heat in those parts, while we sleep, raiseth in the brain an imagination of some kindness shown. In some our dreams are the reverse of our Waking imaginations, the motion when we are awake beginning at one end, and when we dream at another.' - Leviathan; or the Matter, Form, and Power of a Commonwealth, Ecclesiastical and Civil.