A determinative principle of a being. Its form determines, actualises, “informs” the matter. It is a principle of intelligibility, because it is through its form that a being can be known.
More precisely,
the form is what makes a thing what it is: it constitutes its intelligible identity, its essence in act. Without form, matter would be pure undetermination, empty potentiality, incapable of existing or of being thought. The form is not a mere external attribute but the intrinsic principle that actualises matter, delivering it from indeterminacy so as to constitute it as a determined being. Every corporeal reality is thus a composition of matter (potentiality) and form (actuality). It is the form which renders matter knowable: to know is to reach the form.
In the Aristotelian-Thomistic tradition, form (εἶδος, μορφή; forma) is the first act of a being. It confers to matter not only structure but a mode of being; it is both a principle of specification (by which a thing belongs to a species) and a principle of unity (by which it remains the same through change). For example, man is not merely an organic assembly: it is the human form – his soul – which actualises and unifies his corporeality.
The form is not only the informer of matter but also the foundation of operation. Action follows being: because a reality is determined by a form it can act according to that form. Likewise, knowledge results from the form: the intellect knows the abstracted form of things – a form of which it becomes in act, according to the doctrine. This explains the possibility of intellection itself.
The form also manifests the participatory dimension of reality. Every finite form receives its intelligibility from a higher Source: it participates in the first Intelligibility, the exemplary principle of all created form. Thus metaphysics affirms that forms are, in some manner, “expressions” of the Principle. Hence the possibility of a formal hierarchy, ranging from brute matter to the highest spiritual forms, culminating in the divine Intellect.
Form is therefore not a strictly conceptual notion: it structures being, grounds knowledge and connects the created world to its Principle. It is the bridge by which human intelligence reaches the intelligibility of reality.
For further reading
Aristotle, Metaphysics, Z-H (VII–VIII); ed. J. Tricot, Paris, Vrin, 2018 — On the definition of form (εἶδος, μορφή) as principle of act and intelligibility.
Aristotle, De anima, II,1–2; trans. A. Jannone, Paris, GF-Flammarion, 2017 — On the soul as form of the body (μορφὴ σώματος).
Thomas Aquinas, Summa Theologiae, Ia, q. 76-77; ed. Léonine — On the substantial form of the soul and the relation form–matter.
Thomas Aquinas, De ente et essentia, ch. 1–4; ed. É. Gilson, Paris, Vrin, 1948 — On the distinction between form, essence and act of being (esse).
Bruno Bérard, Metaphysics for Everyone (Angelico Press), trans. of Métaphysique pour tous (Paris, L’Harmattan, 2022); ital. Sui sentieri della metafisica; esp. ¿Qué es la metafísica?; all. Was ist Metaphysik? — On understanding the form as principle of intelligibility, its relation to being, and the role of intellect in grasping the formal structure of reality.