Newman terms this spontaneous reasoning faculty the ratiocinative or illative faculty. And he adds that it is actually a collection of faculties adapted to various subject matters, as may be observed in the talents of specialists as different as Newton and Napoleon. Each of us has this illative faculty for natural inference. In some it is a gift characterized by precision, promptitude, and truth; in some it is natural and uncultivated, in some an acquired habit. And it is Newman's opinion that it belongs to women more than to men. In ordinary minds it may be degraded by bias, prejudice, passion and self-interest. The perfection of this faculty Newman terms the illative sense, a use of the word "sense" similar to its use in the terms "good sense", "common sense", or "a sense of beauty". It is right judgment in ratiocination.

Newman sees the path from inference to assent as made up of an objective logical process guided by a subjective personal faculty for discovering truth. The illative sense is the living authority which guides the mind in the discernment of the true limit of converging probabilities. Its role as a guide to truth is parallel to that of conscience, our guide to what is good.

This same illative sense is the living authority which guides the mind in the reflex act of an assent to an assent which, in the case of a true proposition is the basis of certitude. But is there any criterion for the accuracy of an inference in concrete matters? Yes, it is a subjective one. The sole and final judgment on the validity of an inference in concrete matter is committed to the illative faculty. My own illative sense or that of another is my guide to truth in concrete matters. In no class of concrete reasonings, whether scientific, historical, or theological, is there any ultimate test distinguishing truth from error in our inferences, other than the trustworthiness of the sanction of the Illative Sense.