20. The fact of sensation is connected with many others, and from this connection results a great part of our knowledge. It has been said in a tone of great confidence, that it was not possible to demonstrate by sensations the existence of bodies; for as sensations are something purely internal, they cannot enable us to infer the existence of any thing external, and there is no reason for not regarding all our sensations as a collection of individual phenomena, inclosed in our soul. At first view it seems impossible to solve the difficulty; nevertheless, if we examine it thoroughly, we shall see that too great importance has been attached to it. 21. The first objection ordinarily made to the testimony of the senses, is the difficulty of distinguishing with certainty between the state of sleep and that of waking. We receive when asleep impressions similar to those we receive when awake: how shall we know that the illusion is not perpetual? Lamennais, with characteristic exaggeration, says: "He who shall show that all life is not a sleep, an indefinable chimera, will do more than all philosophers have thus far been able to do." There are here, no doubt, grave difficulties; but we cannot persuade ourselves that they are insolvable. First of all we shall examine if sleep and waking be different, not only in the eyes of common sense, but also in those of reason. Lamennais pretends that only at the tribunal of common consent can a satisfactory and definitive sentence be obtained: we are convinced that very close reasoning can arrive at the same result to which consciousness, common sense, common consent, or, in other words, the testimony of our own being and that of our fellow mortals, conjointly conduce. 22. Man finds in himself a perfectly satisfactory certainty of the difference between sleep and waking: we need no testimony of others to know that we are awake. The difference between these states must not be solely sought in the clearness and vividness of sensations, and the certainty which they generate. Undoubtedly images are sometimes presented to us in sleep with as much clearness as if we were awake, and our certainty for the instant is complete. Who has not in sleep experienced great joy, or terrible anguish? Sometimes, but very rarely, we have, when we awake, the reminiscence of having in the very act of sleep doubted if we were asleep; but this seldom happens, and it is in general true, that even our dreams are not accompanied by this twilight of reflex reason which warns us of our state and of the illusion that we are under. Ordinarily, while we dream, we have no thought that we are asleep, and we embrace a friend with the same effusion of tenderness, or weep disconsolately over his tomb, with the same affections as we should were all real.