42. Matter and Form. -- Although it belongs primarily to physics (50, 2), the theory of matter and form assumes a metaphysical meaning, in so far as it is an explanation of motion or change in general. In the essence of all being that is subject to change, we must find: (1) a potential principle, indeterminate as such, which becomes actual by the process of change; or, in other words, a fixed substratum which successively receives contrary determinations; this is matter;^{2} (2) a principle which determines this amorphous substratum, and which is of a special kind for each actuation of the matter; this is the form.

The theory is, of course, primarily and properly applicable to corporeal, terrestrial substances; but Aristotle extends the concept of matter and form even to mathematical entities, and to the heavenly bodies. Not only does he make it co-extensive with the notion of change; he generalizes the notions connoted by matter and form so far as to apply them to everything that is determinable on the one hand or determining on the other; e.g., to the genus as compared with the species, to body as compared with soul, to the passive intellect as opposed to the active, to the premisses in relation to the conclusion. The theory of matter and form becomes, in fact, convertible with the theory of potency and act. "Esti d' men hule dunamis, d' eidos entelecheia (De Anima, ii., 1). Matter is potency or potentiality; form is act. Let us see what is their nature and what sort is the bond which unites them.

As a constitutive principle of being, form gives the composite entity its specific determinateness; it makes the thing to be what it is (to ti ên einai). It accounts for all that is actual perfection in a being, its organization, its unity; in the composite being it is above all the principle of the operations of that being, and as these all tend towards an end (44, 4), it is consequently the seat of the impulse directing the activities of the being. It alone being knowable, it is the sole object of definition.

The matter fulfils various functions. Undetermined itself it is unknowable in itself, and we know it only by analogy. It is because the indeterminate cannot exist that the indefinite (apeiron) does not exist. This characteristic of indeterminateness or of absolute potentiality is the object of one single, homogeneous concept of matter; but there are, in fact, as many different matters as there are beings. While the form is the principle of unity and the seat of impulse towards an end, matter is subject to multiplication and division (43); it is connected with what is fortuitous and teratological; it is, in general, the principle of limitation, of imperfection and of evil. Motion being eternal (44, 3), matter must be likewise eternal. "Generation could not have had a beginning nor can it have an end; because the reason why there is generation is ever identical with itself, and is of abiding efficacy."^{3} Individuals pass away, but the species has always been and will remain for ever.

In the physical order, matter and form are real elements of being; matter is not, then, the on of Plato. Form and matter constitute but one reality owing to their very close union. Form is immanent in matter (against Plato) and can no more free itself from matter "than the roundness can from a ring".^{4} Similarly, matter cannot exist without form: the concept of matter. existing as such, is that of a being, determined, yet undetermined, which is self-contradictory.

Outside the physical order, in whatever domain they are employed, the two notions of matter and form are bound together by the same close correlation.

{1} Physics, iii., 201 b.

{2} On this twofold conception of the matter in Aristotle, cf. CL. BAEUMKER, Das Problem der Materie in der Griechischen Philosophie (Münster, 1900), pp. 213 sqq., and p. 257.

{3} PIAT, op. cit., p. 28.

{4} Ibid., p. 29.