In philosophic inquiry about the validity of knowl edge, some rule or standard, called a criterion (Gr. κριτήριον), by which truth can be known and distinguished from error is usually sought and applied. The fact that men hold different opinions about things, or that sometimes one finds that he has been mistaken in a previous judgment, urges a consideration of how to distinguish between valid knowledge or truth and mere thought or opinion. The very fact that one does distinguish between truth and error indicates that some criterion exists for judging the validity of knowledge. The study of such a criterion is sometimes called criteriology.
Early History. In the history of greek philosophy, the conflicting views of the Ionians, who admitted change in things, and of the Eleatics, who denied it, brought forth the problem of the relationship between sense knowledge and intellectual understanding. socrates was unable to solve questions concerning the nature of changing things and turned more hopefully to the study of moral problems. These he tried to elucidate by processes of induction and definition that, when successfully brought to term, he accepted as genuine knowing, not mere opinion. plato likewise thought that man cannot have genuine knowledge of the sensible world but only more or less probable opinions about it; he held that the realities that are the objects of philosophical inquiry exist in a world of Forms, or Ideas, apart from sensible things.
Aristotle tried to close the gap between the sensible and the intelligible by admitting that intelligible forms are in sensible things and are not separated from them save by the operation of man's understanding. Sensible things are themselves intelligible, but they are known in different ways by the senses and by the intellect. By sense man apprehends the particular and the changing, whereas by intellect he understands the necessary and universal aspects of things. His intellect, working with the data of sense, grasps the intelligible aspect of being and proceeds to understand by distinguishing being from nonbeing, this from that, one from many, whole from part, and cause from effect. The validity of all knowledge can be judged with truth and certitude by the intellect's reflecting on the various kinds of knowledge and evaluating these in the light of the principle of contradiction, which it applies with the help of experience and logical analysis. The chief root of error in man is not the senses but rather the phantasm, which exhibits to the intellect things that are absent as if they were present, and things that are different as if they were alike.
The Stoic philosophers, both Greek and Roman, did not maintain a distinction between sense and intellect, and in effect reduced all knowledge to sensation, with all its vagaries and uncertainties. The Skeptics went a step further and admitted only internal phenomena. They denied all intelligible essences and necessary truths and held that there is no criterion by which one can distinguish between contradictory views, so that man cannot know whether he possesses truth or not.
Scholasticism. Scholastic philosophers admitted a distinction between sense and intellect and held that man is already in possession of many truths, which he attains not only from ordinary experience and the various scientific disciplines but also through faith in the Christian teachings. For the scholastics the chief epistemological problem was to account for the validity of abstract or universal knowledge in relation to the singulars that exist in nature. Some held that the universal is only a word or name (nominalism); others maintained that it expresses something apart from sensible things, either entirely apart (Platonic realism) or separated from sensible matter by the abstractive action of the mind (moderate realism). According to the account of moderate realism, the nature, or essence, that is known by the intellect is really in the singulars that exist in the world of things; but the abstract or universal manner in which it is known depends upon the mind of the knower. (see universals.)
Modern Thought. The modern critical problem arose with the philosophy of R. descartes; it came from his method of universal doubt and from his starting point in thought (cogito ergo sum ). This approach brought up the question whether and how one knows the external world or anything that is real and distinct from one's knowledge of it. Descartes tried to solve this problem with the criterion of clear and distinct ideas, passing from the idea of a limited and imperfect self and the idea of something unlimited and perfect to the assertion of God and His goodness and then to the general veracity of the created mind and the validity of its ideas. G. W. leibniz likewise wished to proceed from thought to things and employed the postulate that each mind mirrors the universe more or less adequately according to a harmony preestablished by God.
I. Kant also acknowledged a gap between thought and things, which he declared unbridgeable in theory, although in practice he maintained that man ought to live as if he possessed valid knowledge of moral duty and its practical implications of freedom, justice, God, and immortality. Kant held that whereas sensible phenomena are particular and contingent, yet they are understood in ways that are determined by the structure and laws of the mind, which molds and shapes sensory experience according to its own patterns. These a priori patterns give universality and necessity to experimental knowledge, and suffice, Kant thought, to account for the scientific character of mathematics and physics. But one has no assurance that things are as he understands them. Indeed things in themselves are unknowable, and man has no valid knowledge of essences or necessary reasons of being. (see criticism, philosophical; noumena; phe nomena.)
The Problem. In summary, philosophers who admit only sense knowledge and neglect or deny intellectual knowledge propose no criterion other than sensation by which to distinguish between truth and error, and they acknowledge the uncertainties and limitations of this criterion. On the other hand, philosophers who admit intellectual knowledge as distinct from sense knowledge find difficulty in relating sense and intellect. Some deny that the sensible is intelligible and assert either that the intelligible is something real and apart from the sensible or that the intelligible is merely the mold or pattern according to which the mind understands sensible phenomena. Others advance in the direction of subjectivism and idealism and deny the distinction between knowledge and its object, that is, between knowledge and things, and maintain that the mind actively constitutes or constructs or posits all its objects, without question of truth or of a criterion of truth.
A Realist Solution. According to natural or moderate realism, the sensible is also intelligible and is understood by the intellect in the light of the apprehension of its own object, namely, being, and the evident first principles and laws of being. For valid knowledge there must be a starting point or principle that is evident and unshakable, and this is found to be the principle of contradiction: it is impossible to affirm and deny at once the same of the same. This principle is so evidently valid that no one can be mistaken about it. Moreover, it is not a supposition or postulate, or a dogmatic affirmation, but something that the mind must and naturally does embrace. Thus equipped with the principle of contradiction, and proceeding on the basis of sense experience, man gradually builds up his understanding of things, distinguishing between one and many, being in nature and being in knowledge, whole and part, cause and effect, etc. By reflecting on his knowledge of things in the light of intelligible being, that is, of objective evidence, which is the universal criterion of truth, he can know the conformity between the intellect judging and the thing judged. Thus he can distinguish between truth and error, and can determine the reasons or causes of truth.
See Also: epistemology; gnoseology; knowledge, theories of.
Bibliography: v. sainati, Enciclopedia filosofica, 4 v. (Venice-Rome 1957) 1:1347–48. j. owens, An Elementary Christian Metaphysics (Milwaukee 1963). r. f. o'neill, ed., Readings in Epistemology (pa; Englewood Cliffs, N.J. 1962).
[w. h. kane]