146. Our mind acquires truth, or at least the appearance of truth; that is to say, that in one way or another, it performs those acts which we call perception and sensation. Whether the reality does or does not correspond to the acts of our soul, is at present of no consequence, is not what we now seek: we place the question on a ground accessible to the most skeptical; for even they do not deny perception and sensation; although they destroy reality, they admit appearance.
147. The means by which we perceive truth are of different orders; and this is why truths perceived correspond equally to different orders, parallel, so to speak, with the respective means of perception.
Consciousness, evidence, and intellectual instinct or common sense, are the three means; to which correspond truths of consciousness, necessary truths, and common-sense truths. These are distinct, different, and in many cases unconnected with each other; and he, who seeks to acquire complete and accurate ideas upon matters relating to the first principle of human knowledge, must mark out their limits with great care.
148. That means which we have called consciousness, or the intimate sense of that which passes within us, that which we experience, is independent of all the others. Destroy evidence, destroy intellectual instinct, yet consciousness remains. In order to feel, and to be sure that we feel, and what we feel, we need only experience. If we suppose the principle of contradiction to be doubtful, still it will not shake our certainty that we suffer when we suffer, that we rejoice when we rejoice, that we think when we think. The presence of the act, or the impression at the bottom of our soul, is intimate, immediate to us, and of irresistible efficacy to place us above all doubt. Sleeping or waking, sane or insane, the testimony of consciousness is the same; there may be an error in the object, but there can be none in the internal phenomenon. The lunatic who believes that he counts numberless bags of dollars, certainly does not count them, and in this he is deceived; yet he has in his mind the consciousness of what he does, and in this he is infallible. A man who dreams that he has fallen into the hands of robbers, is deceived as to the external object, but not as regards the act by which he believes it.