Full Idea
Let any man try to conceive a triangle in general, which is neither Isoceles nor Scalenum, nor has any particular length or proportion of sides; and he will perceive the absurdity of all the scholastic notions with regard to abstraction and general ideas.
Clarification
Isoceles and Scalene are particular shapes of triangle
Gist of Idea
We can't think about the abstract idea of triangles, but only of particular triangles
Source
David Hume (Enquiry Conc Human Understanding [1748], XII.II.122)
Book Reference
Hume,David: 'Enquiries Conc. Human Understanding, Morals', ed/tr. Selby-Bigge/Nidditch [OUP 1975], p.155
A Reaction
I think there is a basic error in this. I admit that I can only imagine a particular triangle, but it doesn't follow that I am thinking about one triangle. Ontology/epistemology confusion. I picture a shape while believing the shape to be irrelevant.