The philosophical ‘problem of universals’ is an ancient one, which continues to be debated in the modern-day. The problem concerns the ontological status of properties and relations. To solve the problem we need to address a fundamental question about universals: do they ‘exist’ as mind independent entities, and if so, what is their nature? In order to examine this problem, we need to be clear about the distinction between particulars and universals.
A ‘particular’ is an individual entity such as a specific blade of grass or a specific tree. Particulars are (in most cases) things we can point to, or that can be observed by at least one of the senses. Particulars can exist over time, but they can only be in one place at a time, they are ‘non-repeatable’ entities.
It is necessarily true that all particulars must have at least some properties (or qualities), for example the colour of a blade of grass or the smell of a piece of bark. Particulars also stand in relation to other particulars, for example one tree is to the left of another tree, or one specific tree is taller or older than another specific tree. These properties (e.g. green-ness) and relations (e.g. to the left of) can be seen to be in more than one place at one time, they are repeatable. The abstract noun ‘tree’ can be used to describe many items and is also repeatable.
The philosophical question is how can properties exist in more than one place at one time? If lots of things are green do they all share in the same green-ness? What is green-ness? Does green-ness exist independently of the particular items that are green? These questions extend to relations and abstract nouns. If one tree is taller than another tree, what is the nature of tall-ness? When we talk about ‘trees’ in general (i.e. abstract as opposed to concrete noun) what are we referring to? We can also extend the investigation to cover non-physical abstract nouns such as ‘justice’ and ‘goodness’.
These questions - how individual properties can be shared, how one quality or relation can be the same as another and what it is that we refer to when we use abstract nouns, are referred to as the problems of qualitative identity and resemblance. The solution, at least for some, is to posit entities called ‘universals’.
Linguistically, proper names stand for particulars whereas adjectives, verbs and prepositions stand for universals. Thus ‘London’ (proper noun) is a particular, whereas the adjective ‘green’ in the phrase ‘the green apple’ and the verb ‘like’ in the phrase ‘I like apples’ and the preposition ‘on’ in the phrase ‘the apple on the table’ are all universals. We know what the proper name
stands for, but what do universals
stand for? London
Universals, if they exist as entities, may be thought to be mind-independent. For Plato, they are immaterial ‘Forms’ which are transcendent, they exist in an abstract realm and can only be known by reason. For others, such as Aristotle, they are in the world, they reside in individual items. For Armstrong, universals are features of the world that are instantiated by particulars.
There are therefore a number of competing theories which attempt to solve the problem of universals. By no means all philosophers believe that universals do in fact exist as distinguishable entities, for them the world is made up only of particulars.