57. It is not easy to ascertain how this combination of one sensation with another enables us to judge rightly of external objects; chiefly because the development of our sensitive and intellectual faculties is completed before we can reflect upon it: and thus we find ourselves certain of the existence and properties of things before we have thought of certainty, and much less of the means of acquiring it. 58. But even supposing us, after occupying ourselves with sensations and their relations with objects, to set aside the certainty which we already have, and to act as if we sought it, we can find no one sensation the basis of the certainty of the other sensations. We should meet in that all the difficulties to be encountered in the others. 59. One of the chief difficulties upon this point is to determine the relations of the sense of sight with that of touch, and how far the one depends upon the other. We propose hereafter to examine these questions at some length, and we shall therefore now refrain from entering upon them, as well because they are not of a character to be incidentally investigated, as because whatever their solution, it is not at all opposed to what we shall here establish. 60. It would be of no advantage to us to know that the certainty of all sensations was, philosophically speaking, founded upon that of some one sensation. Every sensation is a contingent, individual fact: how then are we to draw from it light to guide us to necessary truths? No matter under what aspect we consider sensation, it is only an impression received through our organs. We are sure of the impression because it is intimately present to our mind; and its repetition aided by other sensations, whether of the same or another sense, makes us certain of its relations with the object producing it: but every thing is done instinctively, with little or no reflection; and we are always condemned, however much we reflect, to reach a point beyond which we cannot pass, for nature herself there stops us. 61. Far then from finding in any sensation a fundamental fact on which to found a philosophical certainty, we discover a collection of particular and mutually distinct facts, equal, however, so far as the production in us of that security which we call certainty is concerned. It is of no use to decompose man, and reduce him first to an inanimate machine, then allow him one sense, making him perceive different sensations, afterwards grant him another sense, making him combine the new and the old sensations, and so on synthetically to the possession and exercise of them all. These things may do to entertain one's curiosity, to nourish philosophical pretensions, or to give a show of probability to imaginary systems; but they are in reality of little or no use; the evolutions which the observer imagines do not resemble those of nature; and the true philosopher ought to examine what really is, not what is only in his conception.