325. We shall now inquire whether common sense be a certain criterion of truth, whether it be so in all, or in what cases, and what characters it must have in order to be an infallible criterion. Man cannot lay aside his nature: when it speaks, reason will not allow it to be ignored. A natural inclination, simply because it is natural, is in the eyes of philosophy something highly respectable: it is the province of reason and free will not to allow it to go astray. What is natural to man is not always so perfectly fixed as it is in brutes, where instinct is blind as it must be, where there is neither reason nor free will. The exercise of man's natural inclinations is subordinate to his reason and free will, and, consequently, when these are called instincts, the word has a very different meaning from what it has when applied to brutes. What happens in the moral order is also verified in the intellectual. We have not only our heart to watch, but also our understanding; both are subject to the law of perfectibility; the objects which they offer to us are good and evil, truth and error. Nature herself shows us which one we ought to choose, but does not force our choice; life and death are before us, we may select the one we please. 326. There is in man, independently of the action of free will, a quality which oftentimes has the effect of turning his natural inclinations from their object; it is weakness. Hence, it is nothing extraordinary for these inclinations to be so distorted as to lead to error instead of truth, and this renders it necessary to determine what characters _common sense_ should have, in order to be an absolutely infallible criterion. 327. We will point out the conditions, such as we conceive them to be, of true and never-erring common sense. _First Condition._--That the inclination be every way irresistible, so that one cannot, even by the aid of reflection, resist or avoid it. _Second Condition._--That every truth of common sense be absolutely certain to the whole human race. This condition follows from the first. _Third Condition._--That every truth of common sense stand the test of reason. _Fourth Condition._--That every truth of common sense have for its object the satisfaction of some great necessity of sensitive, intellectual, or moral life. 328. When possessed of all these characters, the criterion of common sense is absolutely infallible, and may defy skeptics to assign a case wherein it has failed. The higher the degree in which the conditions are satisfied, the more certain will the criterion be. We will explain this by a few examples.