 Mathematics also presents issues about what counts as a criterion, even when the definition is quite precise. Two irrational numbers x and y are identical if they have exactly the same relations to each of the rational numbers. Pi is identical with 3.14… because Pi is greater than 3, 3.1, 3.14, etc and less than 4, 3.2, 3.15, etc. But does this count as a criterion? It appeals to the less than relation, which is a close cousin to identity itself and it involves relations to similar entities. 