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A principle asserting the unity, consistency, and stability of being, and commonly enumerated among the first principles. In ontology it is expressed: "Every being is determined in itself, is one with itself, and is consistent in itself"; in logic: "What belongs to being must be predicated of it," or, "Whatever is true (e.g., a con cept, a proposition, or a relationship) is, as such, absolutely and always true." Since the notion of identity is closely associated with that of unity, similar distinctions may be applied to both. Thus one may speak of an identity that is real or logical, physical or moral, and numerical or specific (cf. Aristotle, Meta. 1015b 171017a 7; St. Thomas Aquinas, In 5 meta., 89; ST 1a, 11.1 ad 2).

Explanation. The principle of identity is itself an explication of the concept of being. Being denotes positiveness and determination; it also implies coherence and agreement with itself, without which the determination, and consequently being itself, would dissolve. Although explicitly concerned with the unity of being, the principle of identity also contains an implicit reference to the distinction and multiplicity of being.

The algebraic statement of the principle of identity, "A being is a being, or A = A," is somewhat tautological. If the predicate in no way amplifies the subject, or does not fulfill the expectation of the copula, it would seem that no judgment has in reality been expressed. Thus the statement, "A man is a man," really says nothing; its predicate belies the promise of the verb "is." On the other hand, when the principle is expressed "A being is identical with itself, or is one and consistent in itself," the subject is determined as something underlying unity, and therefore the statement asserts something definite and positive.

Various Interpretations. As a metaphysical principle, the principle of identity has been variously interpreted, while as a logical principle it receives rather uniform interpretation. The statement, "A being is identical to itself and is one in itself," usually is interpreted as applying to being in a transcendental sense, that is, with a potential reference to all its possible concrete determinations, but without referring to any one thing in particular. Thus, identity and coherence with itself is said not merely of the one, eternal, and unchangeable Being, but rather of being as such, whether this be one or multiple. In its logical formulation, the principle of identity presupposes no precise concept of real being, and thus does not create serious theoretical disagreements.

Monistic Views. parmenides presumed to deduce a sweeping theory of reality from the concept of being alone: "One must say and think that being exists" [H. Diels, Die Fragmente der Vorsokratiker: Griechisch und Deutsch, ed. W. Kranz, 3 v. (8th ed. Berlin 1956) frg 6]. "Being is without origin, is immortal, and is everything in itself. It was not, and it will not be, because it is complete in itself, one and continuous" (ibid. frgs 78). By reasoning analogous to this, the ancients thought that all reality could be resolved to one identity; the Epicureans and the Stoics understood this in a materialist, the Neoplatonists in a spiritualist sense. A like tendency is at the roots of the various forms of monism in modern thoughte.g., that of G. bruno, B. spinoza, and F. W.J. schelling. Schelling's conception is in fact known as the "philosophy of identity," according to which the diversity and multiplicity manifest in the world is relative and phenomenal and does not mar the absolute unity that is the foundation of being.

Dualism and Multiplicity. A metaphysical dualism within being is, on the other hand, essential to Aristotelian and to Christian thought, with their accent on matter and form, participated and unparticipated being, and the unity and multiplicity of being. This dualism presents a fundamental problem: how to reconcile the multiplicity of entities with the concept of unity. In Christian thought, a solution is furnished in terms of the concept of pure act, infinite and unparticipated in Itself, and at the same time the creative and exemplary principle of participa tion for individual and finite beings. The doctrine of analogy is further proposed as a logical and metaphysical concept that is most useful for an objective understanding of being in its totality. As Aristotle has remarked, "There are many senses in which a thing may be said to be" (Meta. 1003a 33). The concept of being is formally one, but it is not univocal in its concrete manifestations, and therefore does not warrant an absolute resolution into uniform identity. On the contrary, it manifests an immanent tension, in a real as well as in a dialectical sense, toward an infinity of both determinations and forms.

Evaluation. No solution to the metaphysical problem of unity and multiplicity can be deduced simply from the principle of identity. It is impossible for the human mind to determine the absolute structure of being in general, or of concrete beings, by means of a purely transcendental deduction from the concept of being itself. The intellect does not grasp being in its original concreteness by intuition, but rather attains knowledge of being through abstract concepts based on experience. The basic problem of the unity or multiplicity of concrete being is intimately connected with the processes of analysis, interpretation, and deduction, whereby thought is brought to bear on experience.

With regard to its logical and metaphysical import, one may say that the principle of identity is of lesser significance than the principle of contradiction. Its chief contribution is that it accentuates the value of the positiveness that is essential to the concept of being. Because of this positiveness and consistency in itself, being contains, implicitly and virtually, infinite possibilities of differentiation and development. Given the fact of creation, since being is positive, it can (and perhaps must) set in motion, embrace, and stimulate the processes generating the diverse, the relative, and the contrary, all of which it potentially holds within itself. But these processes, which take place in reality and in history on so grand a scale, are determined by the principle of identity only so far as this in turn implies the principle of contradiction. The expansion of the principle of identity into the principle of contradiction is spontaneous and even necessary, since in its implication of diversity and distinction the very concept of identity becomes, at least indirectly, dialectical. For this reason, St. thomas aquinas, following Aristotle, accords a primacy among first principles to the principle of contradiction.

See Also: identity

Bibliography: l. fuetscher, "Die ersten Seins-und Denkprinzipien," Philosophie und Grenzwissenschaft 3:24 (1930). e. meyerson, Identity and Reality, tr. k. loewenberg (New York 1930); "La Notion de l'identique," Essais (Paris 1936). j. marÉchal, Le Thomism devant la philosophie critique (2d ed.) 56168,v.5 of Le Point de départ de la métaphysique, 5 v. (3d ed. Paris 194449). j. m. de estrada, "Reflexión acerca del principio de identidad," Sapientia 8 (1953) 27682. s. vannirovighi, Enciclopedia filosofica, 4 v. (Venice-Rome 1957) 2:121519. r. eisler, Wörterbuch der philosophischen Begriffe, 3 v. (4th ed. Berlin 192730) 1:70013.

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Identity, Principle of

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