The acts of the mind, wherein it exerts its power over simple ideas, are
chiefly these three: 1. Combining several simple ideas into one compound
one, and thus all complex ideas are made. 2. The second is bringing two
ideas, whether simple or complex, together, and setting them by one
another so as to take a view of them at once, without uniting them into
one, by which it gets all its ideas of relations. 3. The third is
separating them from all other ideas that accompany them in their real
existence: this is called abstraction, and thus all its general ideas
are made.
-- John Locke, An Essay Concerning Human Understanding (1690)