Not everything is susceptible of proof; but every criterion stands the test of reason. The criterion of consciousness is a primitive fact of our nature; in that of evidence we discover the indispensable condition of the existence of reason itself; in that of the intellectual instinct by which we make our ideas objective, is found a law of our nature likewise indispensable to the very existence of reason; in that of common sense, properly so called, is the instinctive assent to truths, which, when examined, are seen to be perfectly reasonable; in that of the senses, and in that of human authority, we discover the same thing as in the other cases of common sense, which is a means of satisfying the necessities of sensation, intellectual, and moral life. The criteria do not conflict with, but mutually aid and confirm each other. Neither is reason at war with nature, nor nature with reason; both are necessary to us; both direct us with certainty, although they are both liable to err, since they belong to a limited and very feeble being. 338. The philosophy which considers man only under a single aspect is incomplete, and in danger of becoming false. So far as certainty is concerned, we must bear in mind this last observation: to become excessively exclusive is to place one's self on the brink of error. We may analyze if we will the sources of truth; but we must not lose sight of their connection when examining them separately. To begin by conceiving a system, and then making everything conform to its demands, is to place truth in the bed of Procrustes. Unity is a great good; but we must be satisfied with the measure imposed upon us by nature. We must seek truth by human means, and in proportion to our capacity. The faculties of our soul are subject to certain laws, which we cannot abstract. One of the most constant laws of our being is the necessity of a simultaneous exercise of our faculties, not only in order to become certain of truth, but even to discover it. Man joins the greatest multiplicity with simplicity: his soul is one, is endowed with various faculties, and united to a body of such variety and complication as to be called with much reason a little world. His faculties are in intimate and mutual relation; they exert a continuous influence upon each other. To isolate them is to mutilate, and sometimes to extinguish them. This remark is important, for it indicates the radical vice of all exclusive philosophy. If a man have no sensations, his understanding has no materials, nor has it that stimulus, without which it remains dormant. When God united our soul and body, it was that one might aid the other; wherefore he established that admirable correspondence between the impressions of the body and the affections of the soul, which, therefore, needs the body as a medium, as an instrument, whether the action of the body upon it be supposed to be a true action, or only a simple occasion of causality of a higher order.