Full Idea
If two names refer to the same object, then in any proposition which contains either of them the other may be substituted in its place, and the truth-value of the proposition of the proposition will be unaltered. This is the Principle of Extensionality.
Gist of Idea
If an object has two names, truth is undisturbed if the names are swapped; this is Extensionality
Source
David Bostock (Intermediate Logic [1997], 3.1)
Book Reference
Bostock,David: 'Intermediate Logic' [OUP 1997], p.72
A Reaction
He acknowledges that ordinary language is full of counterexamples, such as 'he doesn't know the Morning Star and the Evening Star are the same body' (when he presumably knows that the Morning Star is the Morning Star). This is logic. Like maths.