  There is also the related, though uncontroversial, Principle of the Indiscernibility of Identicals: if two things are identical, then they share all properties, or  x = y → (∀F)(Fx ↔ Fy).  The combination of these two principles is sometimes called “Leibniz's Law”: two things are identical if and only if they share all properties, or x = y ↔  (∀F)(Fx ↔ Fy). (Sometimes, unfortunately, only the Principle of the Indiscernibility of Identicals is so called.)