IDEA. — The term, of Greek origin (ἰδέα εἶδος, from ἰδεῖν, to see) originally refers to figure, visible aspect, form: ἰδεῖν τῆς γῆς (Plato, Phaedo, 108 d.; cf. Aristotle, Hist. animal., VI, 35, 580 a 28). The corresponding Latin term is species (from spectare, to look; Cicero preferred the term forma: Top., 7, 30; yet by his time the Greek word had already entered Latin without alteration). Perhaps it was from the value of the external figure as a sign and expression of the internal structure of beings that the term could be transferred to denote the “essential form” or “type” of things. This is the fundamental metaphysical-logical meaning that the term (already used by Democritus in the sense of “determination” and “schema”: ἰδέας οχήματα as atomic structures of matter) assumed in Plato and then, with varied applications, in philosophical currents connected with Platonism. In the history of philosophical thought, the term has become, more than most, overburdened with further meanings and systematic references.
In Plato, the idea is the intelligible unity of the many (Rep., VI, 507 b); the ideas subsist in themselves, immutable, ungenerated, incorruptible (Tim., 51 d ff.); incorporeal, they are the true essence (τὴν ἀληθινήν οὐσίαν, Soph., 246 b), the reasons and truth of beings (τὰν ὄντων τὴν ἀληθινήν, Phaedo, 99 e), the exemplars of nature (παραδείγματα, κτῆμα, φύσει), to which all other things are similar and inasmuch as they participate in them (Parm., 132 d). The idea thus serves in the Platonic system as the rational principle and value of things, their essential determination, and is at the same time the object of intuition or vision (here perhaps lies another justification for Plato’s use of the term) of the soul. The ideas not only have ontological value but, in the interpretation given by Aristotle, subsist in themselves outside material and visible reality (cf. Met., I, 6, 9; XI, 1, 4; VI, 14; XII, 9; XIII, 4). Aristotle, while denying such separate subsistence (ibid., VI, 13), preserves the metaphysical value of the idea. The idea (μορφή, δευτέρα οὐσία) is indeed the universal, the one and the same in the many (ἓν καὶ αὐτὸ ἐπὶ πλεόνων: Anal. post., I, 11, 77 a 9), that is, the essential unity common to all individuals, which is real only in them and derived from them: ἐκ τῶν καθ' ἕκαστα ταύτην τὸ καθόλου (Eth. nicom., VI, 12, 1143 b 4). The idea, therefore, is not, as in Plato, an object of intuition but is obtained through a process of abstraction.
In the Platonic-Aristotelian tradition, the ancient and medieval understanding is largely maintained. While reducing ideas to purely subjective-empirical concepts (they are merely the lingering traces of sensible representations: an anticipation of Hume), thinkers still admit a rational principle (ἀρχή), immutable and immanent in nature. Similarly, in the modern era, the idea is not merely the subjective unity of consciousness (as in Kant) but perhaps also an exemplary form of the world (cf. De mundi opificio, 17 sqq., 24-25). For Cicero, ideas are, as in Plato, “formae rerum,” exemplary principles belonging to the purely intelligible order (“cogitatione tantum et mente conceptae” [ideas]), ungenerated, immutable, eternal (De orat., cap. 3). Seneca likewise states: “Idea est eorum quae natura fiunt exemplar aeternum” (Epist., 58).
In Plotinus, ideas are the eternal content of the spiritual act (νοῦς) with which they are identified. They constitute the intelligible world (κόσμος νοητός) of the spirit; and since activity and thought reside as “their own power” in being, ideas are “the form of being and its operative power” (Enn., V, 1, 9, ch. 8-9). For St. Augustine (and similar is the thought of Pseudo-Dionysius: cf. De div. nom., ch. 5), ideas are the “rationes aeternae” existing in God, the exemplary form and cause of all created things: “ideae principales quaedam formae vel rationes rerum stabiles atque incommutabiles... quae in divina intelligentia continentur” (Quæst. 83, q. 46: PL 40, 30). “Eternal and absolute,” ideas transcend individual human consciousness as immutable objectivity (De lib. arb., III, 7). In God, they serve as formal determinations of His infinite truth; thus, participation in ideas implies in human knowledge the actualization of a relationship with absolute truth and therefore with God (cf. De Civ. Dei, XI, 10).
In the early Middle Ages, the Platonic conception of the real subsistence of the idea was revived in the debate over universals (v.). For William of Champeaux, the “species” has true substantial consistency and is found, as an ontological (not merely formal) unity, in the multiplicity of individuals (cf. Abelard, Hist. calamitatum, ch. 2). The further discussion of the problem of universals led to the more mature formulation of the idea as a form preexisting in God (universal “ante rem”), as a reality individuated in particular beings (universal “in re”), and as the universalization effected by the mind from the specific unity of individuals (universal “post rem”; cf. St. Thomas, De ente et essentia, ch. 4; Quodlib., q. VIII, a. 1). Yet more often than not, in the problem of universals, the term used was “species,” “conceptio,” or “genus.” In the great medieval theologians, the term idea retains rather the Neoplatonic-Augustinian meaning of cause and exemplary form in the divine mind. For St. Albert the Great, ideas are “in mente divina secundum quod ars est omnium creaturarum” (Sum. Theol., I, 3, q. 55, a. 2). According to St. Bonaventure, “ideae rerum ante mundi constitutionem in mente Creatoris erant” (Comp. Theol., I, 25). This doctrine is developed by St. Thomas Aquinas: “oportet dicere quod in divina sapientia sint rationes omnium rerum quas... ideas diximus; id est formas exemplares in mente divina existentes” (Sum. Theol., I, 3, q. 44, a. 3, c.; cf. q. 15, a. 1, c.). The idea in general is form, that is, the objective expression of a thing; it functions as an exemplary principle insofar as it directs action, and as a cognitive principle insofar as it is the term by which the thing is given in consciousness: “secundum quod formae cognoscibilium dicuntur esse in cognoscente” (ibid.). In both senses, ideas are in God: as the principle of creative activity, the idea in the divine mind is “exemplar”; as the principle in and through which the divine knowledge of things is realized, the idea is “ratio” (ibid., a. 3).
The second of the two meanings indicated in St. Thomas (the *idea* as a cognitive representation) is already close, though with a different gnoseological meaning, to the modern sense that arrives (in antithesis to the Platonic meaning, for which *idea* was an objective reality in itself) at the identification of *idea* with “mental representation,” distinguished by its in-itself reality. In Descartes, the *idea* is, in a technical sense, “the form by which every thought or content of knowledge is immediately given to consciousness”: *“ideae nomine intelligente cuiuslibet cognitionis formam per cuius immediatem perceptionem eiusdem cogitationis conscius sum”* (Reply to the Second Objections, definition 2). The *idea* is thus the immediate mental form and term by which every content (intelligible-sensible) is given. This content, insofar as it is in the *idea*, is called by Descartes the *objective reality* of the *idea* (ibid., 3). Thus the *idea*, in itself, does not at all put thought in contact with reality in itself, but rather encloses it within the intentional sphere of representation (cf. *Meditations*, III). This concept of the *idea*, which underlies the gnoseological immanentism of modern philosophy and from which the modern sense of idealism (q.v.) is partly derived, is found in Malebranche (the *idea* is the immediate object of the spirit; what is given in the representative sphere of consciousness); in Spinoza (the *idea* is the conception of the mind insofar as it is a thinking substance: *“per ideam intelligo mentis conceptum quem mens format propterea quod est res cogitans”* (*Ethics*, II, def. 3. The *ideas* constitute the objective being of things, existing as a “nexus of causes” in the infinity of divine *ideas*: ibid., prop. VIII, XX); in Leibniz (the *idea* is a pre-existent form in God and in our intellect, and in itself does not imply the real existence of what it represents: *Nouveaux Essais*, III, ch. 4, § 2). The value of the *idea* as a pure mental image is also a fundamental conception of empiricism. In Locke, *idea* is any object of knowledge and thought. The unity embraces all contents of consciousness, both representative (sensations, images, concepts) and affective (feelings). Only in *ideas* does the immediate object of thought consist. This principle becomes rigorous immaterialism in Berkeley; *ideas* are all the objects of our knowledge (Principles, § 25); and since *ideas* exist only in the spirit (ibid., § 22), any of our sensible representations cannot objectify themselves into a material reality existing in itself. Things coincide with *ideas*; empirical reality is nothing but the constant order and succession of *ideas*, impressed upon our spirit by God (ibid., §§ 33–39). Hume, preserving and indeed accentuating the purely immanent meaning of states of consciousness, understands by *idea* the images or representations derived from the original states of consciousness, that is, from impressions (Treatise, I, sect. 1, § 1). The difference between *idea* and impression lies in the varying degree of vividness and force: *ideas* are nothing but the faint reflections of impressions. Rationalism and empiricism thus operate, in the same direction of immanence, the reduction of the contents of consciousness—the former absorbing into the *idea* as rational value the concreteness of experience, the latter dissolving into the *idea* as sensible representation, or associative synthesis of sensible representations, the intellectual-rational values and concepts.
In Kant, *idea* assumes a new technical meaning. It is a representation *a priori* or a transcendental concept of reason, which cannot be referred to the phenomena of experience as their form or category, but rather serves only as the ideal term of the totality of experience: “By *idea* I mean a necessary concept of reason to which no object adequate to it can be given in the senses” (*Critique of Pure Reason*; Transcendental Dialectic, sect. 2; Italian trans. by G. Gentile and G. Lombardo Radice, I, Bari 1940, p. 300). The *ideas*, and first of all the three fundamental ones (world, soul, God), serve only as pure ideal limits or terms of convergence of experience in the absolute.
In post-Kantian immanentist idealism, the *idea* assumes a metaphysical value even more universal than that it had in Platonism. With every pluralism and dualism suppressed, the *idea* becomes the objective force of the spirit that unfolds, as reason, in the world, and is thus understood as the true substance and transcendental unity in which all moments of the real are resolved. This absolute valorization of the *idea*, already present in Schelling (*idea* is the synthesis of the absolute identity of the universal and the particular); “nature is a reflection of the *idea*;” is developed in the Hegelian system. For Hegel, the *idea* is the adequate concept, the true objective, that is, truth as such. “When something has truth, the *idea* for its *idea*, that is, something has truth only insofar as it is *idea*” (*Science of Logic*, Italian trans. by A. Moni, III, Bari 1927, sect. III, p. 242); “Being has attained the meaning of truth insofar as the *idea* is the unity of concept and reality; it is therefore now only what it is *idea*” (ibid., p. 246). It should be noted that for Hegel the identity of the *idea* with itself is the same as the process (ibid., p. 248), that is, the *idea* is in its innermost being dialectical. Consequently, in every moment of the real, by virtue of its immanence in the *idea*, the profound meaning of identity prevails. To idealism may be likened Schopenhauer, for whom the *idea* denotes the great immutable forms of the objectification of the will, that is, of its phenomenal unfolding in the world and in multiplicity: “These great forms stand to individual objects as their eternal forms and models” (*The World as Will and Representation*, I, Italian trans. by P. Savj-Lopez and G. De Lorenzo, Bari 1928, bk. II, § 25, pp. 162–63).
After idealism, not to speak of substantially traditional attitudes (e.g., Rosmini, in whom the *idea* is properly the intelligibility of essences, or, what is the same, their objectivity—and thus, finally, their value as being: *Teosofia*, V, ed. naz., Rome 1939, p. 127, 136), there is the conception of Kantian inspiration, which understands *ideas* as pure theoretical values, devoid in themselves of ontological content: ideals in which the subject rediscovers itself and that thus stand at the horizon of the spirit as the term and goal of realization in the process of thought and action (e.g., H. Rickert: cf. *System der Philosophie*, I, Tübingen 1921, p. 158 sq.).
Correspondence in ontic reality, for example, in dreams, in imagination. But this does not imply the general assumption that the *idea* cannot transcend itself as subjectivity, nor be, with respect to its objective content, an immediate reference to the real. The purely subjective value of the *idea* is a systematic postulate of gnoseological immaterialism, in no way validated by experience. The mental representation is not to be understood, in itself, as the object and term of the cognitive act, but rather as the intentional mediation by which consciousness relates to the real in the various planes of its objectivity. The *idea* is precisely the effective positing in consciousness of this relationship. At the base of the Cartesian-Lockean assumption lies the radical error of having suppressed or ignored the profound discriminations that experience imposes among the different forms and moments of knowing and the corresponding gradation of ontic-existential consistency of the contents of consciousness. The denial of the objective reference of the *idea*, considering it only in its subjectivity, was logically bound to lead to the absolutization of the ego as subject and ultimately as pure act. The actualism of G. Gentile (q.v.) has absorbed into the spirit as act any residual of objectivity that, even in post-Kantian idealism, was still recognized in the *idea*.
Further current usages of the term (e.g., as intention, purpose, opinion, etc.) tend to dissolve its systematic meaning. Instead, the substantivation frequent in Italian of the term “ideale,” and the sense of *i.* as the essential part or core of a system or doctrine—“the religious *i.*,” “the Christian *i.*”—are reconnected to Plato.