If a man, having no sensations, were to think, he would only think as a pure spirit; he would not be in relation with the external world, nor would he be a man in the sense in which we use this word. In such a case the body is superfluous, and there is no reason why it should be united to the soul. If we admit sensations, and abstract reason, man is converted into a brute. He feels, but does not think: he experiences impressions, but does not combine them, for he is incapable of reflecting. Every thing succeeds in him as a series of necessary, isolated phenomena, which indicate nothing, lead to nothing, and are nothing but affections of a particular being who does not comprehend them, nor render to himself an account of them. It is even difficult to say of what kind are his relations with the external world. Arguing from appearances and analogy, it is probable that brutes also make their sensations objective; but ordinarily, their objectiveness differs from ours. Let us take sleep for example. If brutes sleep, and they probably do sleep, as certain appearances seem to indicate, it would not be strange should they not distinguish as we do between waking and sleeping. This supposes some reflection upon acts, some comparison between the order and constancy of some, and the disorder and inconstancy of others; a reflection which man makes even in infancy, and continues to make all his life without adverting to it. When we awake from a deep sleep, we sometimes remain for several moments in doubt if we are asleep or awake: this doubt alone supposes a comparative reflection of the two states. What do we do in order to resolve this doubt? We examine the place where we are; and the fact that we are abed, in the silence and darkness of night, indicates that the previous vision had no connection with our situation, and therefore that we were asleep. Without this reflection, the sensations of sleeping and waking would be connected, and all confounded in one and the same class. The instinct conceded to brutes, but denied to men, shows that reason was given to us in order to appreciate sensations. There are, then, in man no criteria of truth absolutely isolated. They are all in relation with each other, they mutually affirm and complete each other; for we must note that those truths, of which all men are certain, do in some sense rest upon all the criteria.