The completion of this trend of thought is reached in Plato. According to the platonic doctrine of ideas the sensible world, as a world of becoming, can possess no veritable being or truth. It is a world of mere "opinion" (boO), a world of "appearance," whereas truth and knowledge refer only to objects of pure thought which are not subject to change. This "rational" conception of truth, however, was not without its countercurrents in Greek phi losophy. The Sophists deny the possibility of a universal truth valid for all thinking subjects. According to the proposition of Protagoras that "man is the measure of all things," truth is relative and subjective: "for each, his perception is true" (cf. Plato's Theaetus). Out of this opposition, there develops in later Greek philosophy the controversy between the "dogmatic" and the "sceptical" schools. For the former, there is a universally valid "criterion" of truth, which gives to certain concepts and principles absolute certainty.
According to the Stoics this criterion consists in "cataleptic presentation" KaraX7prrucii). It forms a spontaneous achievement of thought, a type of assent (olyyKaraeats), which must be added to the sensible impression in order to make it known. A doubt with reference to the "cataleptic presentation" is not possible : we possess full evidence that the object is repro duced in it as it is in itself. In opposition to this the Greek Sceptics deny that there is any such criterion of "evidence." All supposed "truth" is mere probability. For the absolute objects, the "things in themselves," are entirely unknown to us (iionXa) ; accessible to our judgment are merely the appearances of things, which, however, take entirely different shapes according to the differences of the perceiving subjects and the conditions of the perception itself. The definiteness of reason, of the "Logos," is thus abandoned: concerning any subject there are innumerable assertions each of which may claim to be equally valid. (See Herbertz, Das Wahrheitsproblem in der griechischen Philosophie, Mediaeval scholasticism, in its definition of the concept of truth, refers throughout to Greek philosophy, especially to Aristotle. According to it, truth consists essentially in the agreement of thinking and being, in the adaequatio intellectus et rei (Thomas Aquinas, De veritate 1, 2). But this definition bears now a new content. For the problem of truth belongs no longer exclusively to the realm of logic, to the sphere of pure cognition, but refers essentially to the question of religious certainty. The opposition between "sensibility" and "thought" is now replaced by the oppo sition between "faith" and "knowledge." Religious faith seeks sup port in the intellect (fides quarens intellectum), although it cannot find in it a sufficient foundation but stands in need of another, superior source of certainty. Thus the problem arises as to how
both principles of certainty, how "natural intellect" and "revela tion" stand to one another. Scholasticism, at times, supposes a relation of mere subordination; at times, it asserts a complete separation between them. In the later nominalism this idea of separation is predominant. There arises the doctrine of "twofold truth," according to which a definite sentence may be true from the standpoint of knowledge and false from the standpoint of I faith. The problem of truth and of logical and religious certainty had been conceived most profoundly by Augustine. "Do not turn outward ; go back into yourself ; in the inner life of man resides the truth." The inner life gives us, as a first indubitable starting point, the certainty of the thinking, feeling and willing subject : the certainty of our own Being and Knowing and Willing. This certainty cannot be attacked by doubt because doubt itself is an act of thought and must, therefore, presuppose the latter. "Who ever knows himself as doubting, knows something true and is certain of that which he knows ; he is thus certain of truth. There fore, whoever doubts whether there is a truth, has thus in himself a truth about which no doubt is possible. Therefore, he who doubts at all, cannot doubt truth as such." (Augustinus, De very religiose, cap. 39.) However, this immanent truth as exhibited in the thinking consciousness, is not the only one, nor a sufficient one for the foundation of religious cognition. For the Being to which religious cognition refers, lies, as an infinite Being, beyond the limits of human consciousness. For a knowledge of the Divine mere ratio must be "transcended" by another power of the mind. Entering into the inner life of the subject and going beyond this inner life are thus the two phases which disclose the realm of truth as a whole. If there were only a truth of finite and contingent things, we could stop with the finite, empirical consciousness as the bearer of this truth. However, since there is a region of eternal truths (veritates aeternae)—Augustine cites the truths of religion and of pure mathematics—there must be an infinite intellect which thinks them and in which they subsist. Human thought does not create these truths ; it is only able to receive them in so far as it is enlightened by a higher power. In every cognition of a necessary and eternal truth, our reason is only seemingly self-active and autonomous. In order to apprehend such a truth, it must be enlightened by the divine intellect. The divine Logos is the "hid den sun" which alone makes truth visible for us.