COMPOSTO. – C. is that reality in which parts can be distinguished. It may be an aggregate of several ontologically distinct and independent realities (accidental c.) or, instead, an ontologically one being (metaphysical c.). Understood in this latter sense, c. may consist of essential parts (e.g., soul and body in the human c.) or integral parts (e.g., the organs that compose the human body). Scholastic terminology further distinguishes essential parts into physical parts (for example, matter and form as the constitutive principles of the body) and metaphysical parts (for example, corporeity and sensibility in the human c.).
The second Kantian antinomy is based on the concept of c. and of the simple, arising from the difficulties that the pre-critical Kant had discerned in the concept of the continuum and believed to be insoluble (v. CONTINUO). In the thesis, which represents the rationalist theory (Leibniz), Kant asserted: “every composite substance in the world consists of simple parts, and nowhere exists anything other than the simple and that which is composed of it” (Critique of Pure Reason, Italian translation, Bari 1945, p. 354); and in the antithesis, which represents the empiricist theory (Hume): “No composite thing consists of simple parts, and nowhere in the world is there anything simple” (ibid., p. 355). The thesis is erroneous because, as can be seen from the demonstration Kant gave of it, it is based on the false presupposition that c. is always an aggregate of distinct beings. The antithesis is true if it is referred to extended c., which cannot consist of simple entities; but there also exist non-extended composite beings. It is also false that “nowhere in the world is there anything simple.”
In experimental science, c. is that body which, when suitably treated, separates into specifically different bodies, and is distinguished by this characteristic from the element, which does not permit such further specification. Modern chemistry distinguishes approximately 90 elements, catalogued according to Mendeleev’s table. Atomic and nuclear physics has further discovered that every body, including the elements, is composed of subatomic corpuscles: proton, neutron, electron, positron, and perhaps some others. The theory of quantum mechanics teaches that these particles are to be regarded as corpuscle-waves. These scientific discoveries today make the problem more acute as to whether the composite body should be conceived as an aggregate of elements (corpuscles) or as a single substance. Theoretically, if the composite body is an aggregate of elements, these are found in it as distinct and individual entities; if, on the other hand, c. is a single substance, the elements are not actually distinct within it, since a substance, insofar as it is one, cannot result from an aggregate of substances. If c. is ontologically one, the elements exist in it potentially or, as it is said, virtually, insofar as c., although being one, by virtue of its origin from several elements, is constituted in such a way as to possess, in its various parts, properties similar to those of the component elements, and to allow, upon disintegration, the reappearance of the elements that entered into its formation, and in the same proportion (Proust’s law).
To determine how, in the inorganic world, the elements exist in c., one must consult experience. Modern physics interpreted experience by orienting itself toward the schemas of mechanical metaphysics (one need only recall Bohr’s theory, according to which the atom is a mechanical system of electrons revolving around the nucleus) and assumed that the elements exist actually in c., considering all bodies, with the exception of primordial particles, as more or less stable aggregates of corpuscles bound together by various forces. But the most recent physics (quantum mechanics) interprets experience differently, rejects mechanical schemas, and tends instead to regard atoms and other complex bodies as unitary systems in which the components have lost their individuality. Present-day physics is therefore inclined to consider composite bodies, at least some of them (the atom, molecule, etc.), as substantial unities in which the elements exist virtually.