Metaphysics

One central notion in metaphysics (understood in a broad sense, and going back to before the word was invented) is that of form. Plato (c. 428–348/7 B.C.E.) reasoned that there could be no knowledge of the objects of everyday experience, but merely opinion about them, because they are constantly changing and cannot be said to be, but merely to become. The objects of knowledge, he thought, must be unchanging things—what he called "Forms" or "Ideas." Although Platonic Forms can be grasped only through the powers of the intellect, not by the senses, they are by no means merely mental entities, but independently existing realities, as a result of participation in which the objects of sense perception have the characteristics they do—so, for example, beautiful things are beautiful by virtue of participating in the Form of Beauty (or, as Plato sometimes says, in Beauty Itself). Beyond this, Plato gives rather different accounts of the Forms from dialogue to dialogue, and even within the same dialogue. In perhaps his most famous discussion of them, in the central books of the Republic, he presents a supreme Form, the Form of the Good, which is understood only after years of intellectual training. The other Forms, Plato says, depend on the Form of the Good and are grasped properly only once the Form of the Good is grasped. In the Timaeus, Plato shows the universe being constructed as an ensouled living thing, according to the pattern of the Ideas.

Aristotle's account in his Metaphysics of how things are constituted also uses the notion of Form, but it is treated very differently. The Forms Aristotle discusses are those of types of natural things, divided according to natural kinds. For instance, one natural kind is Man, another is Horse. According to this hylomorphic account, each man is a man by virtue of the Form of humanity, and each horse a horse by virtue of the Form of equinity. These Aristotelian Forms are not, like Plato's Forms, independent single entities in which many particular sensible things participate. Rather, a particular member of a natural kind—this man, for instance—is a concrete whole composed of matter and Form. Yet the Form that makes him a man is the same as the Form by which any other man is a man. By grasping this Form with our intellects, we have a universal notion of Man, which we are able to use in formulating scientific truths, not about this or that man, but about Man in general.

In the Middle Ages, Plato's Theory of Forms in its pure state had very few adherents. But an adapted version of it, which goes back to Philo (c. 13 B.C.E.–45/50 C.E.), the Hellenistic Jewish thinker, was extremely popular: the Forms are said to be in the mind of God. Thinkers as diverse as Augustine, Eriugena, Abelard, and Aquinas used this idea as one of the ways of describing God's relation to created things. It was not until William of Ockham (c. 1285–?1349), in the early fourteenth century, that this picture was seriously questioned. Adoption of this version of Platonic Ideas still left Aristotelian hylomorphism as the main way of explaining the constitution of concrete things, and there was a vigorous debate in the twelfth century about whether (on the basis of the version of Aristotle's theory proposed in his Categories) Forms in reality are really universal, or are universal merely in the way they are used in human thought and language. In the thirteenth and fourteenth centuries, hylomorphism was the basis (following Aristotle's On the Soul) for the theory of intellectual cognition. The Form that makes, for example, a horse a horse informs the matter-like potential intellect of the person who intellectually grasps the notion of horse.