The intimate connection of the problem of truth with the problem of God, which is characteristic of mediaeval epistemology, continues for a long time also in modern philosophy. The after effects of the Augustinian doctrine are obvious in Nicolaus Cu sanus and Marsilius Ficinus. (See under FiciNus.) (Cf. Ernst Cassirer, Individuum and Kosmos in der Philosophie der Renais sance, 1927.) But also Descartes and Malebranche, Cudworth and the Cambridge Platonists show clearly traces of this. On the other hand, however, there develops a more and more pronounced tend ency to emancipate the problem of truth from all entanglements with theological questions and to give it logical independence. Modern rationalism, as represented especially by Herbert of Cher bury's De veritate (1624), maintains the pure self-determination, the unconditioned autonomy of thought. Human reason decides according to its own, universally valid, innate principles about truth and error. Its fundamental concepts and axioms are abso lutely binding. There can be no conflict between faith and knowl edge because any truth of faith needs the test of reason to attain to and be determined in its certainty. In Descartes, Spinoza and Leibniz, the clear and distinct cognition (clara et distincta per ceptio) is set forth as the criterion of truth. Those judgments alone may claim universal assent which are founded on clear and distinct ideas and which allow us to recognize the connection of these ideas with the same certainty that obtains in the propositions of mathematiCs. (See RATIONALISM.) Accordingly, the characteristic trait of truth, the norms veri tatis, is no longer placed in the agreement of a presentation or cognition in us with an external "transcendent" object, but in the agreement with definite principles of reason itself. Every idea is called true which can be derived from them in a strictly deductive manner, by the method of purely logical or mathematical infer ence. When this proof has been given for a certain idea, there can be no doubt that the objective reality must conform with the idea; for this agreement forms, itself, a necessary axiom of reason. Thus, Spinoza, for instance, expounds in his Treatise on the im provement of the understanding that that which makes a thought a true thought, must lie entirely within it ; that the real cause of truth is to be sought, therefore, not in an external object, but exclusively in the intellect itself. The mathematical idea of the sphere, for instance, is "true" even if there is no entity to be found in intuition and empirical perception which corresponds to the definition of the sphere in all respects and with perfect accuracy; and after this ideal conception of the sphere, we determine the reality of "nature" as it is grasped and "clearly and distinctly" recognized through geometry and mechanics. With particular clarity this conception of truth appears in Leibniz's Meditationes de cognitione, veritate et ideis (1684). Against the nominalistic view of Hobbes, that truth consists only in the correct connection of certain linguistic signs (veritas non in re, sed in dicto consistit ) , Leibniz emphasises that the correctness of the connection between the signs must be based on a relation between the ideas them selves. Truth, therefore, always depends on the relations which subsist between the pure ideas.
The process in which the assumption of one idea necessarily implies another, gives birth to the systematic coherence of knowl edge—and this nexus according to the principle of sufficient reason makes up the fundamental character of all truth. Even the truth of phenomena consists only in the fact that they obey certain laws which correspond to the ideal truths of pure logic and pure mathematics. A different form is given to the problem of truth in the English philosophy of the 17th and 18th centuries. In Locke the "intuitive" truth of mathematical ideas is contrasted with the empirical certainty of the existence of things, which can never be inferred by pure logic, but rests on the testimony of sense percep tion. Empirical truth, in this sense, belongs only to the simple
ideas which are given to us immediately through sensation or reflection. What we can predicate, on the other hand, about the connection of these simple ideas, about their coexistence, their regular recurrence, etc., has no truth in the strict sense of the word, no demonstrative certainty, but rests on mere probability. With Berkeley and Hume, this view is developed further and stated more radically with reference to the empirical knowledge of nature. The order of nature is not founded on principles which can be understood in themselves as necessary and uncon ditionally valid, but it represents a purely empirical regularity with which we become acquainted through habit and custom; the knowledge of this order can, therefore, never claim to be more than probable.
With Kant, the battle between the rationalistic and the empiri cal conceptions of truth is decided in such a way that the laws of pure understanding are recognized as necessary and universal, as valid a priori, but that, on the other hand, these laws themselves can retain their validity only within the bounds of experience. The traditional definition of truth—namely, that it is the "agree ment of knowledge with its object"—involves, as Kant expounds in the Critique of Pure Reason (p. 82), a logical circle. For the problem of knowledge consists in determining just what the "rela tion of a presentation to its object" means, and how we can ascertain this relation. The concept of an "object of knowledge" is only the concept of a "Something in general= X," "because, apart from our knowledge, we have nothing which we could set over against it as corresponding to it." "It is clear, however, that because we have only to do with the manifold of our presenta tions and because that X which corresponds to them, in so far as it is to be something distinct from our presentations, is for us nothing, the unity made necessary by the object can be nothing other than the formal unity of consciousness in the synthesis of the manifold of presentations. Then we say : we know the object, when we have effected synthetical unity in the manifold of intu ition." (Kritik der reinen Vernunft, 1st ed., p. 104 seq.) The "transcendental" truth, the truth of the object, rests thus on the "formal" truth ; for only the forms of pure intuition and under standing make a connection of the phenomena possible; this con nection, however, and nothing else, is meant when we attribute to a phenomenon objective significance, objective validity. How ever, since, on the other hand, the concepts of the synthesis a priori, although not originating in experience, all refer, never theless, to experience as the condition of its possibility, one may say that the possibility of experience is that which alone gives objective reality to all our cognitions. Cognition a priori has, there fore, truth (agreement with the object) only because it contains nothing further than what is necessary for the synthetic unity of experience. (Kritik der reinen V ernunft, 2nd ed. p. 195 ff.) In this sense, Kant declares that with regard to theoretical cog nition of nature, experience supplies the rule, and that it is the "source of truth" (Krit. d. r. Vern. p. 375) but in contradistinc tion to the empiricism of Locke and Hume, experience is here to be understood not as a "mere rhapsody of perceptions," but a synthetical unity according to laws. In this point, Kant sees the decisive contrast between his "formal" idealism and the specu lative idealism represented, for instance, by Berkeley. "The thesis of all true idealists, from the Eleatic school down to Bishop Berkeley, is contained in this formula : 'All cognition through sense and experience is sheer illusion, and only in the ideas of pure understanding and reason is truth.' The principle which governs and determines my idealism throughout, is, on the contrary : 'All cognition of things merely out of pure under standing or pure reason, is sheer illusion, and only in experience is truth' " (Prolegomena, Appendix).