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Introductory Paragraph

Early Ionic Natural Philosophers

The Pythagoreans

The Eleatics


Later Natural Philosophers

General Character of the First Period in the History of Greek Philosophy

The Sophist


The Followers of Socrates

The Lesser Socratics

Plato. Life. Works

Plato. Philosophy

The Disciples of Plato

The Old Academy

Aristotle: Life and works

Aristotle: Theory of Knowledge

Aristotle: Metaphysics

Aristotle: Physics

Aristotle: Psychology

Aristotle: Practical Philosophy

Aristotle: Rhetoric and Poetic

Aristotle: Sources

Aristotle: Unity of Plato and Aristotle

Aristotle: result

The Peripatetic School

Three Leading Post-Aristotelian Schools

The Stoics and Stoicism

The Epicureans and Epicureanism

The Sceptics

The Common Ground of the Stoics, Epicureans, and Sceptics

Philosophy in Rome: Eclecticism

The Later Peripatetics

The Later Academics

The Later Stoics

General Character of the Second Period

Standpoint and Schools of the Third and Latest Period of Greek Philosophy

Jewish-Alexandrian School


The Eclectic Platonist

Neo-Platonism. Plotinus

Neo-Platonism. Porphyry. Jamblichus

Neo-Platonism. Proclus




B. C. BURT (1852-1915) - Table of contents                        




§ 14 - Aristotle

Aristotle's Theory of Knowledge

We begin the exposition of Aristotle's philosophy with an account of what Aristotle himself regarded as introductory to philosophy proper, viz., his theory of knowledge, of its sources and method.


Kinds of Knowledge

There are in knowledge three fundamental differences that Aristotle takes cognizance of in his theory of knowledge: differences as regards the object, method, and source of knowledge. Knowledge may have for its object causes (or first principles) or phenomena. Its method may be apodictic (demonstrative) or dialectic ("probable"). Its source may be sense or reason.


Scientific, or Philosophical Knowledge

Scientific, or philosophical, knowledge (έπιστήμη) is knowledge, the subject of which is causes (άρχαί), the method, demonstration (άπόδειξις), and the source, reason (νοϋς). In the knowledge of causes is involved the knowledge of whatever else can be shown demonstratively to flow from them ; and a theory of scientific knowledge is an account of the source from, or faculty by, which we get the knowledge of causes, and of the method of demonstration. Now it was natural, both from the previous history of speculation and the character of the problem itself, that Aristotle should consider the latter part of the problem of scientific knowledge first, which he does, particularly in the Prior Analytics.


Considered as regards method, knowledge is scientific, or demonstrative, when it is derived from certain, or necessary, premises by a certain, or necessary, process of reasoning.

The Syllogism (Deductive)

Now the process in which, certain things being assumed as true, a certain other thing obtains, necessarily and because of the things assumed, is called by Aristotle the Syllogism(1). The syllogism is, therefore, the central point in the method of demonstration expounded by Aristotle; it was regarded, and rightly so, as his own discovery(2). The syllogism (συλλογισμός) consists of three propositions : two premises (προτάσεις) and a conclusion (συμπέρασμα) ; and has three terms (όροι), the major or larger term (μεϊζον άκρον), the minor or smaller term (έλαττον άκρον), and the middle (μέσον), which may be larger in compass than either of the others or between them. The middle term is so called because it is the mean, or uniting term, in the syllogism(3).—The members of the syllogism are propositions. Propositions may be either affirmative or negative, universal, particular, or indefinite. There are four sorts of propositions that may enter into the syllogism : the universal affirmative, the universal negative, the particular affirmative, the particular negative. As regards the relations of these,—the universal affirmative and particular negative are opposed as contradictories (άντιφατικώς άντικείσθαι); so are the universal negative and particular affirmative. The universal affirmative and the universal negative are contrarily opposed (έναντίως άντικεϊσθαι). Of contradictory opposites, if one be true, the other is false: contraries may both be false(4).—Now the rules of the syllogism given by Aristotle are, that in every syllogism there must be one affirmative premise, there must be one universal premise, and terms must not be treated as universal in the conclusion which are not so in the premises(5).—Sylogisms differ in kind and scientific value according to the relative compass and the position of the middle term. A syllogism in which the minor term is "in the whole middle" (i.e., is the subject of a proposition of which the middle is predicate) and the middle term is "in the whole major" (i.e., is the subject of a proposition in which the major is predicate) is termed a syllogism of the "first figure" (πρώτον σχήμα). In a syllogism of this figure the middle term lies "between" the extremes. A syllogism in which both major and minor terms are "in [less than] the whole middle (i.e., are subjects of propositions in each of which the middle term is a predicate), is a syllogism of the second figure (δεύτερον σχήμα). A syllogism in which the major and minor terms are each greater than the middle (i.e., are predicates of propositions in each of which the middle term is subject) is a syllogism of the third figure (τρίτον σχήμα)(6). (The "fourth figure" of modern text-books was not recognized by Aristotle; it did not spring out of his conception of the syllogism.) Now the first figure is the only one that gives universal conclusions; the second figure giving only negative conclusions, and the third only "particular" conclusions. It is also the only figure that yields naturally and directly in the conclusion all that is contained in the premises and no more. We can sometimes derive a universal conclusion from the premises of a syllogism of the second and third figures, but this can be done only indirectly; hence these figures are "imperfect," the first alone being "perfect."(7)—The hypothetical syllogism (συλογισμός έξ ύποθέσεως) is a syllogism in which, there being a certain condition, a certain proposition obtains. The latter, however, may obtain when the former does not ; but if the latter does not obtain, the former does not(8).

Definition and the Predicables

Now the conclusion of a syllogism the premises of which are true, states, if the conclusion be a universal affirmative and is correctly drawn, a scientific truth, and is virtually the expression of the essence (ούσία), or nature, of some real existence ; hence is a definition.


  The knowledge of the essence of a thing embraces a knowledge of the common and characteristic attributes of the class, or genus, to which it belongs and of the specific attribute that renders the thing an individual representative of the class. In other words, the definition (όρος) contains the expression of the union of the genus (γένος) and the differentia (διάφορα) of the thing defined. Other attributes (not necessary to definition, however) are the property (ϊδιον), which, though essential, is not a mark of distinction, and the accident (συμβεβηκός), which may or may not belong to the subject defined.

The Categories(9).

Essence, or substance (ούσία), is one of the ten aspects, according to Aristotle, under which things in general must be viewed. Substance is whatever is the subject of attributes, e.g., man, Socrates ; and it is either an individual, a species, or a genus. Substance in the primary sense is the individual, species and genus being only secondary substance(10). The remaining nine aspects are quantity (πόσος = how many?), quality (ποιός = of what kind?), relation (πρός πι), place (ποϋ), time (ποτέ), position (κεϊσθαι), condition (έχειν), action (ποιεϊν), passivity (πάσχειν). Of these ten—termed categories—substance is principal; all others imply it. These are everywhere employed by Aristotle. (The idea of a table of categories may have been suggested to him by the Pythagorean table (see The Pytagorean Phylosophy, Theories not purely Pythagorean).) These categories were not "deduced" in any manner from a higher conception by Aristotle, but were taken empirically, as suggested, perhaps, by the fundamental forms, or "parts," of speech in the Greek language.

Syllogism (Inductive)

Now causes (in the knowledge of which or of what can be syllogistically shown to flow from them scientific knowledge consists), though visible to the eye of reason, are not known to us immediately. Knowable things are of two kinds: those which are prior for us (πρός ύμάς πρότερον) and those which are prior by nature (φύσει πρότερον). Of the latter-named kind are causes, or first principles(11). Our knowledge of causes, or what is prior by nature, has its beginning in our knowledge of things as they are for us. The (syllogistic) process by which we reach those firm universal propositions which state the essence, or are the definitions, of causes is induction (έπαγωγή), which is the " passing from particulars to universals," and is the inverse of deduction, which is the passing from universals to particulars(12). Induction, like deduction, is syllogistic: for in induction we unite by inference the middle term to one of the extremes (major and minor terms) by means of the other. Thus, if B is a "middle" to A and C, we can prove by means of C that A may be predicated of B. For example, let the deductive syllogism be, "B is A, C is B, therefore C is A"; then the inductive syllogism is, "C is A, C is B, therefore B is A". The conclusion of the inductive syllogism corresponds to the major premise of the deductive. The inductive syllogism is a syllogism of the third figure, and strictly speaking, its conclusion is not universal but particular. We may, however, assume it to be universal if we know that C and B are inter-convertible and that "B is C" holds. (The syllogism is then practically a syllogism of the first figure.) A real induction presupposes a knowledge of all the individuals of a class. "No particular kinds of Induction are formulated by Aristotle, but he has noticed incidentally the principle of most of the 'Experimental Methods', and in particular that of the method of concomitant variations"(13). The premises of the inductive syllogism are not truths of reason, corresponding to first principles, but perceptions of sense. But sense as such gives knowledge only of the particular, and we can by induction reach universals only on the hypothesis that there is a common and permanent nature in the many. The idea of a common permanent nature originates in a higher faculty than sense. Animals have the faculty of sense-perception, but not all animals have the power of "retaining one certain thing in the soul" and of forming universal notions. Permanency and universality presuppose reason. Aristotle, however, allows himself to say that sense-perception introduces, or "informs" (έμποιεϊ), the universal(14). (He compares the manner in which the universal unconsciously grows out of the particular of sense to the way in which soldiers in battle are caused to fly by the perception of one, and then another, and so on, fleeing.) In so far, however, as there results from the inductive syllogism something that is not given in sense-perception as such (and it is by induction only that we reach the first universals that are the foundation of science), induction does not prove (άποδείκνυσιν) anything (it is not άπόδειξις) though it does show (δηλοϊ) something.

Probable Proof ; Dialectical and Rhetorical Method

The foregoing is, in outline, Aristotle's account of scientific method as employed particularly in speculative, or theoretical, philosophy. He recognizes and gives a full analysis of another sort of method, which is only quiasi-scientific and finds place especially in Practical Philosophy, —ethics, politics, etc. Here "dialectical," or probable, reasoning is employed. In practical affairs it generally suffices if we have premises that possess only a high degree of probability, and if our conclusions have, not absolute validity, but a fair warrant in the premises. In such matters it is not always easy or even possible to arrive at absolutely correct definitions, and it is not always necessary that all steps in our processes of reasoning should be stated, that everything should be proved, even plausibly; indeed, it is better that many things be taken for granted, that many things be left to the natural bent of mankind towards truth and justice. This is the case particularly in rhetorical argumentation; in dialectical reasoning logical method prevails though the premises may be only plausible(15). In rhetorical reasonings the enthymeme, a quasi-syllogism, having but one premise, and example, by which we argue from a particular to a particular (through an assumed universal), may be employed instead of the complete syllogism and induction.


(1) Prior Analytics, I. I. The reader cannot do better here than to follow the account of Aristotle's logic given in Wallace's Outlines of the Philosophy of Aristotle.

(2) Soph. Elench, ch. 33.

(3) Prior Analytics, Bk. I. chs. 25, 1, 4, 5, 32, 26, 6 (see Wallace's Outlines, pp. 37-39).

(4)On Interp. 6; Cat. 10; Pr. Analyt. I. ch. 2, etc. (Wallace, pp. 29-31).

(5) Prior Analytics, Bk. I. ch. 24 (Wlllace, p. 40).

(6) Prior Analytics, Bk. I. chs. 5, 24, 26, 32, 56 (Wallace, pp. 37-39).

(7) Ibid., Bk. I. chs. 1, 5, 7, 23, 24, etc. (Wallace, pp. 39-41).

(8) Ibid., Bk. II. 4 (Wallace, pp. 41-42).

(9) Categories, 4, 7, 8; Topics, Bk.. I. 9; Metaphysics, Bk. VI. 1, 7 (Wallace, pp. 25-27).

(10) Aristotle's doctrine of substance, as we shall see in what follows, appears inconsistent or at least undeveloped (see Metaphysics, Bk. VII. ch. 7). The above view is the earliest, and the one that seems to harmonize best with the theory of categories, in which it occurs.

(11) Posterior Analytics, Bk. I ch. 2.

(12) Topics, Bk. I. 12, 18; Prior Analytics, Bk. II. 23 (Walllace, pp. 42-44).

(13) Wallace, p. 43; see Prior Analytics, Bk. II. ch. 23.

(14) See below, pp. 144-146.

(15) It is a part of Aristotle's catholicity of temper that he shows some fondness for this quasi-scientific method. Absolute truth, he repeatedly says, is not in all cases within the reach of human powers, and it is often-times necessary and best to be content with less than that. If we deny that this is in any sense a philosophical view, we must throw away his works on Ethics and Rhetoric. See below, pp. 148 and 181.



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