Full Idea
When two numbers are so combin'd, as that the one has always a unit answering to every unit of the other, we pronounce them equal.
Gist of Idea
Two numbers are equal if all of their units correspond to one another
Source
David Hume (Treatise of Human Nature [1739], I.III.1)
Book Reference
Hume,David: 'A Treatise of Human Nature', ed/tr. Mossner,Ernest C. [Penguin 1969], p.119
A Reaction
This became known as Hume's Principle after Frege made use of it for logicism (Foundations §63). It reduces equality to something fairly simple and visual (one-to-one correspondence). But we also say that two logicians or musicians are 'equal' in ability.