Full Idea
Universals arise solely from the fact that we avail ourselves of one idea in order to think of all individual things that have a certain similitude. When we understand under the same name all the objects represented by this idea, that name is universal.
Gist of Idea
A universal is a single idea applied to individual things that are similar to one another
Source
René Descartes (Principles of Philosophy [1646], I.59)
Book Reference
Descartes,René: 'Philosophical Essays and Correspondence', ed/tr. Ariew,Roger [Hackett 2000], p.246
A Reaction
Judging by the boldness of the pronouncement, it looks as if Descartes hasn't recognised the complexity of the problem. How do we spot a 'similarity', especially an abstraction like 'tool' or 'useful'? This sounds like Descartes trying to avoid Platonism.