INDIVIDUATION, PRINCIPLE OF. - Existing beings in the world, while sharing a fundamental commonality in their specific nature, are singular and individuated: the principle of individuation is that from which the individual determination of the nature or specific essence fundamentally derives.
The investigation of the principle of individuation has been a much-debated problem in philosophy, especially in the medieval period. It «addresses the essential question of philosophy, and is not merely a topic for intellectual exercises, as the problems that preoccupied medieval philosophers are usually considered» (G. Gentile, *Teoria generale dello spirito*, 4th ed., Florence 1924, p. 57). However, the problem has lost much of its importance in modern philosophy; Spinoza still addresses it within his pantheistic conception of reality, whereby the individual is merely a mode of the single divine substance. Leibniz, in his early *Disputatio metaphysica de principio individui* (1663), resolves the problem in a nominalistic sense by simply affirming that each thing individuates itself by its entire entity (sua tota entitate individuatur). A further reference to the problem appears in Kant’s dissertation (*Principiorum primorum cognitio metaphysicae nova dilucidatio* [1755]). With the rise of immanentist idealism, the problem of the principle of individuation loses the meaning it had in the realistic conception of traditional philosophy, since in idealism the plurality of individuals is absorbed into the unity of the transcendental Ego, though attempts are still made to explain the multiplicity of the empirical in dependence on space and time. Indeed, Schopenhauer explicitly posited space and time as the principle of individuation: «for through space and time what is one in being and concept appears instead as diverse, as a juxtaposed and successive plurality» (*Die Welt als Wille und Vorstellung*, vol. II, § 23; Italian trans. by P. Savi Lopez and G. De Lorenzo, vol. I, Bari 1928, p. 143 ff.). In existentialism (v.), the problem of the principle of individuation is considered rather from a psychological standpoint, and its solution is indicated in the situation (family, homeland, era, etc.) through which each person is inserted into reality and from which they receive their individuation. Philosophically, however, the ontological aspect of the problem remains more significant, though obscure and difficult, so much so that Bossuet despaired of being able to resolve it (*La logique*, chap. 23).
To better evaluate the various solutions, it is useful to distinguish the different ways in which the principle of individuation can be understood. First, it can function as a mere principle of manifestation of the individual: in this sense, it consists of the complex of qualities by which individuals are empirically distinguished; for example, in the case of a human being: name, surname, parentage, place and date of birth, and more or less profound somatic characteristics such as stature, facial features, color (of eyes, hair), voice timbre, and fingerprint. The complex of these determinations is generally sufficient to distinguish two living beings of the same species (though not always, as in the case of twins). It is less easy, however, to determine the distinctive characteristics of individuation in inorganic beings, for example, in two molecules of water. And yet these too are individually distinct and therefore presuppose a constitutive principle of their individuality.
In the search for the constitutive principle of individuation, the efficient cause from which every effect receives its singular reality—and thus its individuality—has sometimes been indicated. But it must be observed that the efficient cause, being extrinsic to the effect, is not solely the extrinsic principle of individuation, and the problem regarding the intrinsic principle of individuation remains unresolved, since every being must have within itself (even if received from a cause) the reason for its own individuality. Nor can this intrinsic principle of individuation be understood as the complete essence of the individual thing (F. Suárez, *Disp. Met.*, d. 5, s. 6), precisely because the principle of this individuality is what is being sought; nor as something distinct from it in reality (Scotus’ *haecceitas*: *In I Sent.*, d. 3, q. 6), because every thing is essentially singular or individual, and therefore its intrinsic principle of individuation must be found within its very essence.
Aristotelian-Thomistic metaphysics (Aristotle, *Met.*, XI, 8; St. Thomas Aquinas, *De ente et essentia*, chap. 2) identifies matter as the intrinsic constitutive principle of individuation for bodies. Certainly spiritual substances too are individuals, but for them the problem does not arise, because, according to the Aristotelian-Thomistic principles of limitation and multiplication of act through potency (v. ACT AND POTENCY), spiritual substances, not being received in a potency, cannot be multiplied within the same species but are individuals in themselves; each individual is therefore a species, and individuation coincides with specification. This holds all the more for God, whose essence, being identified with the pure act of being, is absolutely immutable and thus in itself individual and singular. The problem is thus confined to material substances (and accidents), which admit a multiplicity of individuals within the same species. Such multiplicity is possible, according to St. Thomas, insofar as act—that is, specific perfection—is received in a potency or subject, namely in matter; accidental act in substance, and it is multiplied and individuated according to the multiplication and individuation of matter. While specification comes from act—that is, from form (a human being is such by virtue of the soul)—individuation comes from matter (this is this particular human being by virtue of matter).
Matter, however, is not the principle of individuation as prime matter or pure potentiality, but as endowed with quantity; for matter without quantity has no extension and thus no parts, is neither this nor that, that is, is not individual and cannot therefore individuate form or act. Instead, by virtue of quantity, matter is extended and consequently has parts that distinguish themselves from one another by their different position in space (ex situ), being this or «that», that is, individual, and can thus individuate form. Therefore, this material substance is this individual because it has this essence, composed of this matter and this form; and the form is «this» because it is received in this matter which is «this» because it is this determined part in the totality of corporeal matter. «Materia signata» is thus the intrinsic constitutive principle of individuation for corporeal substances.
Around the question of “materia signata,” the scholastics debate whether the signatio or individuation of matter is due to the order it bears to quantity or to quantity itself actually informing the matter. The problem arises from the fact that matter cannot be informed by quantity unless it already possesses its own act or substantial form, prior to every accident; on the other hand, matter cannot individuate form unless it is itself individuated. Hence some scholastics, following GESPA (Comment. in De ente et essentia, s. Th. Ag., cap. 2), conclude that since matter must be individuated before receiving quantity in order to individuate form, the individuation of matter is not brought about by quantity actually informing the matter, but by the order to quantity that is in prime matter prior to quantity itself.
Yet others, with the Ferrarese (Comment. in Sum. c. Gent. I, 21) and in accord with the opinion of CAPREOLUS (Defens. theol. d. Th. Ag., t. III, ed. C. Paban & Th. Pèques, Tours 1902, pp. 200 ff.), rightly observe that the priority of substantial form over accidental form or quantity is a priority of nature, not of time; so that in the same instant matter, form, and quantity concur together to constitute the new being: form specifying, quantity marking the matter, and matter limiting and individuating the form.
This solution encounters no difficulty in the case of the human soul, which, being immortal, subsists after death as an individual and singular entity even when not united to matter. For the human soul, being spiritual, has its being independent of the body; and so, although created in the body and individuated by it, it preserves its subsistent being and likewise its individuation, remaining always that very form ready to reconstitute the same individual when (philosophy may ignore this, but faith teaches it) it is reunited with the body at the final Resurrection.