The principle that the number of things with the property F equal the number of things with the property G if and only if there is a one-to-one correspondence between those that are F and those that are G. The attribution to Hume derives from the discussion in Treatise, i. 3. 1: ‘When two numbers are so combined as that the one has always an unit answering to every unit of the other we pronounce them equal; and it is for want of such a standard of equality in extension that geometry can scarce be esteemed a perfect and infallible science.’
Philosophy dictionary. Academic. 2011.