Universals in the context of "Existence"

Aristotle's theory of universals is Aristotle's classical solution to the problem of universals, sometimes known as the hylomorphic theory of immanent realism. Universals are the characteristics or qualities that ordinary objects or things have in common. They can be identified in the types, properties, or relations observed in the world. For example, imagine there is a bowl of red apples resting on a table. Each apple in that bowl will have many similar qualities, such as their red coloring or "redness". They will share some degree of the quality of "ripeness" depending on their age. They may also be at varying degrees of age, which will affect their color, but they will all share a universal "appleness". These qualities are the universals that the apples hold in common.

The problem of universals asks three questions. Do universals exist? If they exist, where do they exist? Also, if they exist, how do we obtain knowledge of them? In Aristotle's view, universals are incorporeal and universal, but only exist only where they are instantiated; they exist only in things. Aristotle said that a universal is identical in each of its instances. All red things are similar in that there is the same universal, redness, in each thing. There is no Platonic Form of redness, standing apart from all red things; instead, each red thing has a copy of the same property, redness. For the Aristotelian, knowledge of the universals is not obtained from a supernatural source. It is obtained from experience by means of active intellect.

The Isagoge (Greek: Εἰσαγωγή, Eisagōgḗ; /ˈaɪsəɡoʊdʒiː/) or "Introduction" to Aristotle's "Categories", written by Porphyry in Greek and translated into Latin by Boethius, was the standard textbook on logic for at least a millennium after his death. It was composed by Porphyry in Sicily during the years 268–270, and sent to Chrysaorium, according to all the ancient commentators Ammonius, Elias, and David. The work includes the highly influential hierarchical classification of genera and species from substance in general down to individuals, known as the Tree of Porphyry, and an introduction which mentions the problem of universals.

Boethius' translation of the work, in Latin, became a standard medieval textbook in European scholastic universities, setting the stage for medieval philosophical-theological developments of logic and the problem of universals. Many writers, such as Boethius himself, Averroes, Peter Abelard, Duns Scotus, wrote commentaries on the book. Other writers such as William of Ockham incorporated them into their textbooks on logic.