The Stoics held, as Mr. Herbert Spencer, who resembles them in so many respects, now holds, that all knowledge is ultimately produced by the action of the object on the subject. Being convinced, however, that each single perception, as such, is fallible, they sought for the criterion of certainty in the repetition and combination of individual impressions; and, again like Mr. Spencer, but also in complete accordance with their dynamic theory of Nature, they estimated the validity of a belief by the degree of tenacity with which it is held. The various stages of assurance were carefully distinguished and arranged in an ascending series. First came simple perception, then simple assent, thirdly, comprehension, and finally demonstrative science. These mental acts were respectively typified by extending the forefinger, by bending it as in the gesture of beckoning, by clenching the fist, and by placing it, thus clenched, in the grasp of the other hand. From another point of view, they defined a true conviction as that which can only be produced by the action of a corresponding real object on the mind. This theory was complicated still further by the Stoic interpretation of judgment as a voluntary act; by the ethical significance which it consequently received; and by the concentration of all wisdom in the person of an ideal sage. The unreserved bestowal of belief is a practical postulate dictated by the necessities of life; but only he who knows what those necessities are, in other words only the wise man, knows when the postulate is to be enforced. In short, the criterion of your being right is your conviction that you are right, and this conviction, if you really possess it, is a sufficient witness to its own veracity. Or again, it is the nature of man to act rightly, and he cannot do so unless he has right beliefs, confirmed and clinched by the consciousness that they are right.