Axiom:Leibniz's Law/Historical Note

Historical Note on Leibniz's Law
used this law as the definition of equality.

However, notes:
 * To regard Leibniz's law here as a definition would make sense only if the meaning of the symbol "$=$" seemed to us less evident than that of expressions [such as 'every property $x$ has, $y$ has'].

Hence Leibniz's law can also be adopted as an axiom, or not adopted at all.