Form (Greek μορφή / εἶδος, Latin forma) is the determining principle of a being: that whereby something is what it is (τὸ τί ἦν εἶναι). It is the counterpole to matter (ὕλη) and, together with it, constitutes the hylomorphic principles of constitution of the being.
Aristotle develops the doctrine of form in the Metaphysics (Book VII) and in De anima: the soul is the form of the body — that which makes the living body a living body. Thomas Aquinas takes up this thought and deepens it: the human soul (anima intellectiva) is the substantial form of the human being. It is what makes the human being into what he essentially is — a rational living being.
For the question of personhood the concept of form is fundamental. Personhood is not an external property that can accrue to or fall away from the human being, but belongs to the essential form of the human being himself. Whoever is human is a person — from fertilization onward, in the First Dimension (prōtē energeia). The form determines being, not the faculties actually exercised. The principle agere sequitur esse — action follows being — expresses precisely this connection: the Second Actuality (acts performed) follows upon the First Actuality (being-as-form), not the reverse.
Form and matter are ontologically distinct: the form is not the matter, the matter is not the form. Yet they are actual only together — as the one, concrete, living being.
Sources: Generated by querying the Personhood ontology.
Further sources:
- Aristotle: Metaphysics, Book VII (Ζ). Edited and translated by H. Bonitz, rev. H. Seidl. Hamburg: Meiner (PhB 307/308), 3rd ed. 1991. (German)
- Aristotle: De anima, Book II, ch. 1. Translated after W. Theiler, ed. H. Seidl. Hamburg: Meiner (PhB 476), 1995. (German)
- Thomas Aquinas: Summa Theologiae, I, q. 76 (De unione animae ad corpus).