Aristotle's theory of universals is one of the classic solutions to the problem of universals. Aristotle thought—to put it in a not-very-enlightening way—that universals are simply types, properties, or relations that are common to their various instances. In Aristotle's view, universals exist only where they are instantiated; they exist only in things (he said they exist in re, which means simply "in things"), never apart from things. Beyond this Aristotle said that a universal is something identical in each of its instances. So all red things are similar in that there is the same universal, redness, in each red thing. There is no Platonic form of redness, standing apart from all red things; instead, in each red thing there is the same universal, redness. The problem of universals is a phrase used to refer to a nest of intertwined problems about universals within the philosophy of language, cognitive psychology, epistemology, and ontology. ... Aristotle (Greek: AristotélÄ“s) (384 BC – March 7, 322 BC) was a Greek philosopher, a teacher of Plato and of Alexander the Great. ... Universals (used as a noun) are either properties, relations, or types, but not classes. ... A type is a category of being. ... In metaphysics (in particular, ontology), the different kinds or ways of being are called categories of being or simply According to the Aristotelian tradition, a being is anything that can be said to be in the various senses of this word. ... In metaphysics (in particular, ontology), the different kinds or ways of being are called categories of being or simply According to the Aristotelian tradition, a being is anything that can be said to be in the various senses of this word. ... Instantiation may be Philosophy: A concept in Platonism, see idea Instantiation principle - the idea that if properties exist, the essence that has the properties must necessarily exist Universal instantiation and existential instantiation, two rules of inference in logic Instantiation (computer science), A concept in object-oriented programming; see Object (computer...

To further flesh out Aristotle's theory of universals, it is useful to consider how the theory might satisfy the constraints on theories of universals listed in the problem of universals article. The problem of universals is a phrase used to refer to a nest of intertwined problems about universals within the philosophy of language, cognitive psychology, epistemology, and ontology. ...

First of all, on Aristotle's view, universals can be multiply instantiated. Aristotle stresses, after all, the one and the same universal, applehood (say), that appears in each apple. Common sense might detect a problem here. (The problem can arise for other forms of realism about universals, however.) Namely, how can we make sense of exactly the same thing being in all of these different objects? That after all is what the theory says; to say that different deserts, the Sahara, the Atacama, and the Gobi are all dry places, is just to say that the exact same being, the universal dryness, occurs at each place. Universals must be awfully strange entities if exactly the same universal can exist in many places and times at once, or so one might think. But maybe that's not so troubling; it seems troubling if we expect universals to be like physical objects, but remember, we are talking about a totally different category of being. So a common defense of realism (and hence of Aristotle's realism) is that we should not expect universals to behave as ordinary physical objects do. Maybe then it is not so strange, then, to say that the exact same universal, dryness, occurs all over the earth at once; after all, there is nothing strange about saying that different deserts can be dry at the same time. This article or section does not cite its references or sources. ...

Are Aristotelian universals abstract? And are they, then, what we conceive of when we conceive of abstract objects such as redness? Perhaps. It will help to explain something about how we form concepts, according to Aristotle. We might think of a little girl just forming the concept of human beings. How does she do it? When we form the concept of a universal on Aristotle's theory, we abstract from a lot of the instances we come across. We as it were mentally extract from each thing the quality that they all have in common. So how does the little girl get the concept of a human being? She learns to ignore the details, tall and short, black and white, long hair and short hair, male and female, etc.; and she pays attention to the thing that they all have in common, namely, humanity. On Aristotle's view, the universal humanity is the same in all humans (i.e., all humans have that exact same type in common); and this allows us to form a concept of humanity that applies to all humans.

Are Aristotelian universals the sorts of things we refer to when we use general terms, like 'redness' and 'humanity'? Again, perhaps. The idea is that when we refer to humanity, we refer to the type, human being, that appears identically in each human. We do not refer simply to all the humans, but instead the type, human being, which is the same in each human.

See also

• Aristotle
• Epistemology
• Problem of universals