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( Epistéme , knowledge, science, and lógos , speech, thought, discourse).

Epistemology, in a most general way, is that branch of philosophy which is concerned with the value of human knowledge.

The name epistemology, is of recent origin, but especially since the publication of Ferrier's "Institutes of Metaphysics: the Theory of Knowing and Being" (1854), it has come to be used currently instead of other terms, still sometimes met with, like applied logic, material or critical logic, critical or initial philosophy, etc. To the same part of philosophy the name criteriology is given by the authors of some Latin textbooks and by the Louvain School.

The exact province of epistemology is as yet but imperfectly determined, the two main views corresponding to the two meanings of the Greek word epistéme . According as this is understood in its more general sense of knowledge, or in its more special sense of scientific knowledge, epistemology is "the theory of the origin, nature and limits of knowledge " (Baldwin, "Dict. of Philos. and Psychol.", New York, 1901, s.v. "Epistemology", I, 333; cf. "Gnosiology",I,414); or "the philosophy of the sciences ", and more exactly, "the critical study of the principles, hypotheses and results of the various sciences, designed to determine their logical (not psychological ) origin, their value and objective import" ("Bulletin de la Société fran¸aise de Philos.", June, 1905, fasc. no. 7 of the Vocabulaire philosophique, s.v. "Epistémologie", 221; cf. Aug., 1906, fasc. 9 of the Vocabul., s.v. "Gnoséologie", 332). The Italian usage agrees with the French. According to Ranzoli ("Dizionario di seienze filosofiche", Milan, 1905, s.v. "Epistemologia", 226; cf. "Gnosiologia", 286), epistemology "determines the objects of every science by ascertaining their differentiating characteristics, fixes their relations and common principles, the laws of their development and their special methods".

Here we shall consider epistemology in its first and broader meaning, which is the usual one in English, as applying to the theory of knowledge, the German Erkenntnistheorie, i.e. "that part of Philosophy which, in the first place, describes, analyses, examines genetically the facts of knowledge as such ( psychology of knowledge ), and then tests chiefly the value of knowledge and of its various kinds, its conditions of validity, range and limits (critique of knowledge )" (Eisler, Wörterbuch der philos. Begriffe, 2d ed., Berlin, 1904, I, 298). In that sense epistemology does not merely deal with certain assumptions of science, but undertakes to test the cognitive faculty itself in all its functions.


The first efforts of Greek thinkers centre around the study of nature. This early philosophy is almost exclusively objective, and supposes, without examining it, the validity of knowledge. Doubt arose later chiefly from the disagreement of philosophers in determining the primordial elements of matter and in discussing the nature and attributes of reality. Parmenides holds that it is unchangeable; Heraclitus, that it is constantly changing; Democritus endows it with an eternal inherent motion, while Anaxagoras requires an independent and intelligent motor. This led the Sophists to question the possibility of certitude, and prepared the way for their sceptical tendencies. With Socrates, Plato, and Aristotle, who oppose the Sophists, the power of the mind to know truth and reach certitude is vindicated, and the conditions for the validity of knowledge are examined. But epistemological questions are not yet treated on their own merits, nor kept sufficiently distinct from purely logical and metaphysical inquiries. The philosophy of the Stoics is primarily practical, knowledge being looked upon as a means of right living and as a condition of happiness. As man must act according to guiding principles and rational convictions, human action supposes the possibility of knowledge. Subordinating science to ethics, the Epicureans admit the necessity of knowledge for conduct. And since Epicurean ethics rests essentially on the experience of pleasure and pain, these sensations are ultimately the practical criterion of truth. The conflict of opinions, the impossibility of demonstrating everything, the relativity of perception, became again the main arguments of scepticism. Pyrrho claims that the nature of things is unknowable, and consequently we must abstain from judging; herein consist human virtue and happiness. The representatives of the Middle Academy also are sceptical, although in a less radical manner. Thus Arcesilaus, while denying the possibility of certitude and claiming that the duty of a wise man is to refuse his assent to any proposition, admits nevertheless that a degree of probability sufficient for the conduct of life is attainable. Carneades develops the same doctrine and emphasizes its sceptical aspect. Later sceptics, Ænesidemus, Agrippa, and Sextus Empiricus, make no essential addition.

The Fathers of the Church are occupied chiefly in defending Christian dogmas , and thus indirectly in showing the harmony of revealed truth with reason St. Augustine goes farther than any other in the analysis of knowledge and in the inquiry concerning its validity. He wrote a special treatise against the sceptics of the Academy who admitted no certain, but only probable, knowledge. What is probability, he asks in an argument ad hominem, but a likeness of or an approach to truth and certitude? And then how can one speak of probability who does not first admit certitude? On one point at least, the existence of the thinking subject, doubt is impossible. Should a man doubt everything or be in error, the very fact of doubting or being deceived implies existence. First logical principles also are certain. Although the senses are not untrustworthy, perfect knowledge is intellectual knowledge based on the data of the senses and rising beyond them to general causes. In medieval philosophy the main epistemological issue is the objective value of universal ideas. After Plato and Aristotle the Scholastics hold that there is no science of the individual as such. As science deals with general principles and laws to know how far science is legitimate it is necessary to know first the value of general notions and the relations of the universal to the individual. Does the universal exist in nature, or is it a purely mental product? Such was the question raised by Porphyry in his introduction to Aristotle's "Categories". Up to the end of the twelfth century the answers are limited to two, corresponding to the two, possibilities mentioned by Porphyry. Hence if one may speak of Realism at that period, it does not seem altogether correct to speak of Conceptualism or Nominalism in the well-defined sense which these terms have since acquired (see De Wulf, Hist. de la phil. médiévale, 2d ed., Louvain 1905). Later, a distinction is introduced which St. Thomas formulates clearly and which avoids both extremes. The universal as such does not exist in nature, but only in the mind. Yet it is not a mere product of mental activity; it has a basis in really existing things; that is, by their individual and by their common features, existing things offer to the mind a basis for the exercise of its functions of abstraction and generalization. This moderate Realism, as it is called in opposition to Conceptualism on the one side, and on the other, to exaggerated, or absolute Realism, is also essentially the doctrine of Duns Scotus; and it prevailed in the School till the period of decadence when Nominalism or Terminism was introduced by Occam and his followers.

In modern times Descartes may be mentioned for his methodical doubt and his solution of it in the Cogito, ergo sum, i.e. I think, therefore, I exist. But Locke, in his "Essay concerning Human Understanding", is the first to give a clear statement of epistemological problems. To begin with ontological discussions is to begin "at the wrong end" and to take "a wrong coursed." Hence "it came to my thoughts that . . . before we set ourselves upon inquiries of that nature, it was necessary to examine our own abilities, and to see what objects our understandings were, or were not fitted to deal with" (Epistle to the Reader). Locke's purpose is to discover "the certainty, evidence and extent" of human knowledge (I, i, 3), to find "the horizon which sets the bounds between the enlightened and dark parts of things, between what is, and what is not comprehensible by us (I, i, 7), and "to search out the bounds between opinion and knowledge " (I, i, 3). One who reflects on the contradictions among men, and the assurance with which every man maintains his own opinion "may perhaps have reason to suspect that either there is no such thing as truth at all, or that mankind hath no sufficient means to attain a certain knowledge of it" (I, i, 2). This investigation will prevent us from undertaking the study of things that are "beyond the reach of our capacities" (I, i, 4), and will be "a cure of skepticism and idleness" (I, i, 6). Such is the problem; among the main points in its solution may be mentioned the following: "we have the knowledge of our own existence by intuition ; of the existence of God by demonstration; and of other things by sensation" (IV, ix, 2). The nature of the soul cannot be known, nor does the trustworthiness of the senses extend to "secondary qualities"; a fortiori, substance and essences are unknowable. These and other conclusions, however, are not reached by a truly epistemological method, i.e. by the criticism of the processes and postulates of knowledge, but almost exclusively by the psychological method of mental analysis. Following in Locke's footsteps and proceeding farther, Berkeley denied the objectivity even of primary qualities of matter, and Hume held a universal and radical phenomenalism. Aroused from his "dogmatic slumber" by the skepticism of Hume, Kant took up again the same problem of the extent, validity, and limits of human knowledge. This is the task of criticism, not the criticism of books and systems, but of reason itself in the whole range of its powers, and in regard to its ability to attain knowledge transcending experience. Briefly stated, the solution reached by Kant is that we know things-as-they-appear, or phenomena, but not the noumena, or things-in-themselves. These latter, precisely because they are outside the mind, are also outside the possibility of knowledge. Kant's successors, identifying the theory of being with the theory of knowing, elaborated his "Critique" into a system of metaphysics in which the very existence of things-in-themselves was denied. After Kant we reach the present period in the evolution of epistemological problems.


Today epistemology stands in the foreground of philosophical sciences. The preceding outline, however, shows that it was the last to be constituted as a distinct investigation and to receive a special systematic treatment. In older philosophers are found partial discussions, not yet coordinated and regarding only special aspects of the problem. The problem itself is not formulated before Locke, and no true epistemological solution attempted before Kant. In the beginning of philosophical investigation, as well as in the beginning of cognitive life in the individual, knowledge and certitude are accepted as self-evident facts needing no discussion. Full of confidence in its own powers, reason at once rises to the highest metaphysical considerations regarding the nature, essential elements, and origin of matter and of the human soul. But contradiction and conflict of opinions oblige the mind to turn back upon itself, to reflect in order to compare, test, and perhaps revise its conclusions; for contradictions cause doubt ; and doubt leads to reflection on the value of knowledge. Throughout history, also, interest in epistemological questions is aroused chiefly after periods characterized by ontological investigations implying the assumption of the validity of knowledge. As the psychology of knowledge develops problems of epistemology grow more numerous, and their solutions more varied. Originally the choice is almost exclusively between affirming the value of knowledge and denying it. For one who looks upon knowledge as a simple fact, these are the only two possible alternatives. After psychology has shown the complexity of the knowing-process, pointed out its various elements, examined its genesis, and followed its development, knowledge is no longer deemed either valid or invalid in its totality. Certain forms of it may be rejected and others retained; or knowledge may be held as valid up to, but not beyond, a certain point. In fact, at present, one would look in vain for absolute and unlimited dogmatism as well as for pure and complete skepticism. Opinions vary between these two extremes; and hence comes, partly at least, the confusion of terms by which various views are designated--a labyrinth in which even the most experienced can hardly find their way. Here a few systems only will be mentioned, and their names used in their most general and obvious sense.

The main problems of epistemology may be conveniently reduced to the following.

  • Starting from the fact of spontaneous certitude, the first question is: Does reflection also justify certitude? Is certain knowledge within man's power? In a general way Dogmatism gives an affirmative, Scepticism a negative answer. Modern Agnosticism attempts to indicate the limits of human knowledge and concludes that the ultimate reality is unknowable.
  • This leads to a second problem: How does knowledge arise, and what modes of knowledge are valid? Empiricism admits no other trustworthy information than the data of experience, while Rationalism claims that reason as a special faculty is more important.
  • A third question presents itself: What is knowledge ? Cognition is a process within the mind with the special feature of referring to something without the mind, of representing some extramental reality. What is the value of this representative aspect? Is it merely the result of the mind's inner activity, as Idealism claims? Or is the mind also passive in the act of knowing, and does it in fact reflect some other reality, as Realism asserts? And if there exist such realities, can we know anything about them in addition to the fact of their existence ? What is the relation between the idea in the mind and the thing outside the mind ? Finally, even if knowledge is valid, the fact of error is undeniable; what then will be the criterion by which truth may be distinguished from error ? What signs decide whether certitude in any ease is justified? Such systems as Intellectualism, Mysticism, Pragmatism, Traditionalism, etc., have attempted to answer these questions in various ways.
  • Like all other sciences, epistemology should start from self-evident facts, namely the facts of knowledge and certitude. To begin, as Descartes did, with a universal doubt is to do away with the facts instead of interpreting them; nor is it possible consistently to emerge out of such a doubt. Locke's principle that "knowledge is conversant only with our ideas " is contrary to experience, since in fact it is for the psychologist alone that ideas become objects of knowledge. First to isolate the mind absolutely from external reality, and then to ask how it can nevertheless come into contact with this reality, is to propose an insoluble problem. As to the Kantian attitude, it has been criticized repeatedly for examining the validity of knowledge with the knowing faculty, for making reason its own critic and judge while its lights to criticize and judge are still held in doubt. Epistemology, the science of knowing, is closely related to metaphysics, the science of being, as its necessary introduction, and as gradually leading into it. The main epistemological issue cannot be met without stepping almost immediately on metaphysical ground, since the faculty of knowledge cannot be examined apart from its exercise and therefore from the contents of knowledge. Logic in its strict sense is the science of the laws of thought; it is concerned with the form, not the matter of knowledge, and in this it differs from epistemology. Psychology deals with knowledge as a mental fact, apart from its truth or falsity ; it endeavours to determine the conditions, not only of cognitive, but of all mental processes and to discover their relations and the laws of their sequence. Thus logic and epistemology complement the work of psychology in two different directions, and epistemology forms a transition from psychology and logic to metaphysics. The importance of epistemology can hardly be overestimated, since it deals with the ground-work of knowledge itself, and therefore of all scientific, philosophical, moral, and religious principles. At the present time especially it is an indispensable requisite for apologetics, for the very foundations of religion are precisely the doctrines most frequently looked upon as beyond the reach of human intelligence. In fact much recent discussion concerning the value of knowledge has taken place on the ground of apologetics, and for the distinct purpose of testing the value of religious beliefs. If, contrary to the definitions of the Council of the Vatican, the existence of God and some at least of His attributes cannot be demonstrated, it is evident that there is no possibility of revelation and supernatural faith. As Pius X expresses it (Encycl. "Pascendi", 8 Sept., 1907), to confine reason within the field of phenomena and give it no right and no power to go beyond these limits as to make it "incapable of lifting itself up to God and of recognizing His existence by means of visible things. . . . And then all will readily perceive what becomes of natural theology, of the motives of credibility and of external revelation ". (See SCEPTICISM ; CERTITUDE; DOUBT.)

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