Metaphysical Form in Ancient and Medieval Thought

Moved by this and a number of other arguments, Aristotle (384–322 B.C.E.) famously rejected Plato's Forms. Hence the adage attributed to Aristotle, usually quoted in Latin: amicus Plato sed magis amica veritas—I love Plato, but I love the truth even more. Aristotle did not abandon, however, what has been called above the individualized forms of things. For Aristotle, it is these forms that individually determine each thing's nature, sorting them into their natural kinds, reflected in the Aristotelian system of categories.

Individualized forms are either substantial or accidental. Substantial forms are what account for the individual existence of substances, the independently existing individuals that are not properties of other individuals, such as this man. Accidental forms (or briefly, accidents) are the individualized properties of substances, such as the height or color of this man. Individual substances and accidents are classified in the system of categories into their universal kinds, their species and genera.

But how can individuals have universal knowledge concerning all individuals of the same species and genera if they cannot have the direct acquaintance with their universal exemplars presumed by Plato? Aristotle's answer is his theory of abstraction. In his view, the human intellect is not a merely passive recipient of "ready-made" universal information obtainable from the universal exemplars of particulars, but it is capable of actively extracting this universal information from the experience of several particulars by separating what is common to all and disregarding what is peculiar to each. The human soul, the individualized substantial form of a human body, uniquely has this ability among other living bodies, through the so-called active intellect (nous poietikos, Gr.; intellectus agens, Lat.). This unique ability of the human soul ranks it the highest in perfection among all material forms, linking it to the realm of immaterial forms or separate substances. In fact, some commentators of Aristotle, such as Averroës (Ibn Rushd; 1126–1198), interpreted Aristotle as claiming that the active intellect itself is such a separate substance. Other, more perfect separate substances are the movers of celestial spheres that by the light of the celestial bodies they hold provide the influx of energy required for the activity of the natural agents on earth (mixing the four elements, and sustaining the generation and corruption of living things). However, Aristotle argues that it is impossible to go in the series of ever more perfect movers moved by even more perfect ones to infinity. So, ultimately all are moved by a first unmoved mover, the Prime Mover, which is therefore the source of the activity of everything else.