Full Idea
Leibniz's Law says 'x = y iff for all P, Px iff Py'.
Clarification
'Iff' means if and only if; Px means x has the property P
Gist of Idea
Leibniz's Law says 'x = y iff for all P, Px iff Py'
Source
Colin McGinn (Logical Properties [2000], Ch.1)
Book Reference
McGinn,Colin: 'Logical Properties' [OUP 2003], p.7
A Reaction
That is, two things are the same if when we say that one thing (x) has a property (P), then we are saying that the other thing (y) also has the property. A usefully concise statement of the Law.