EVIDENCE. - From e-videre (Gk. ἐνέργεια) it is, in a general sense, the clarity and manifestation of the true. The etymological meaning of the word leads to a more precise determination of the real content of the concept; since the clarity of truth is in the end nothing but the necessity of judgment, evidence may be defined as the objective necessity with which a judgment imposes itself upon thought. Everything and only that which imposes itself upon the mind as necessarily thinkable (by absolute or hypothetical necessity), by force of its objective conceptual determination, or of its concrete presentation in experience, is evident. Understood in this way, evidence, although it always implies, as a relative concept, a relation to the knowing subject, nevertheless asserts itself as an essentially objective value.
It must therefore not be confused either with certainty or with the clarity of perception (Descartes), both of which can be illusory and independent of the truth and necessity of the judgment. Likewise, and consequently, it is only in relation to intellectual knowledge that one can speak of evidence in the proper sense; so-called empirical and aesthetic evidence is formally not such, except insofar as the datum of sensible or aesthetic perception is taken up into the assertive act of the intellect, in which alone the formal value of truth is realized.
Evidence may be found both in primary judgments (rational principles and intuitive judgments of fact) and in derived ones; but in the latter evidence too is mediated, insofar as it is obtained through the process of demonstration that shows its connection with truths and principles evident in themselves. One then speaks of intrinsic and extrinsic evidence, according as the necessity of the judgment is grasped in the very relation between subject and predicate, or is instead known indirectly in the extrinsic value of authority (evidentia veritatis, evidentia auctoritatis). Only intrinsic evidence is evidence in the formal sense.
Traditional gnostology, following Aristotle in this, the Epicurean and Stoic school and St. Thomas, places in evidence the universal criterion of certainty, in the sense that it is its necessary and sufficient condition. To know is in fact to perceive, to intuit: for this spiritual intuition to be possible, an indispensable condition is that the object open itself in its intelligibility to thought; and it is also a sufficient condition, because the mind cannot fail to pass to assent as soon as it has grasped the intelligibility and truth of the object. Nor is a further criterion properly needed to discern true evidence from so-called apparent evidence: the latter, precisely because it does not posit itself as absolute necessity or thinkability of the object, can always be overcome by means of a rigorous mental discipline and a full spiritual freedom that does not yield to moving on a purely subjective order.
It must be noted, however, that evidence, as a criterion of truth, cannot claim greater value than the very notion of a criterion of truth entails (v. CRITERION). Truth, because of the complexity of forms and levels according to which it is distributed in human knowledge, in dependence on the respective complexity of objects, cannot be governed by a single abstract and theoretical criterion. Evidence must therefore always be seen and judged in the objective-subjective concreteness with which it presents itself in the individual consciousness, where the subjective and relative factors of individual personalities often have a more or less significant impact. Evidence thus in the concrete comes to coincide, in a certain sense, with truth itself and is subject to the same demands and apparent antinomies that the problem of truth entails in some of its aspects (v.).