Full Idea
At least since Russell, one has routinely distinguished between the 'is' of predication ('Socrates is wise', Fx), the 'is' of identity ('Morning Star is Evening Star', =), and the 'is' of existence ('the cat is under the bed', Ex).
Gist of Idea
There are the 'is' of predication (a function), the 'is' of identity (equals), and the 'is' of existence (quantifier)
Source
José A. Benardete (Metaphysics: the logical approach [1989], Ch. 7)
Book Reference
Benardete,José A.: 'Metaphysics: The Logical Approach' [OUP 1989], p.44
A Reaction
This seems horribly nitpicking to many people, but I love it - because it is just true, and it is a truth right at the basis of the confusions in our talk. Analytic philosophy forever! [P.S. 'Tiddles is a cat' - the 'is' membership]