Full Idea
My concept of an elm tree is exactly the same as my concept of a beech tree (I blush to confess). ..We still say that the extension of 'elm' in my idiolect is the same as the extension of 'elm' in anyone else's, viz. the set of all elm trees.
Clarification
An 'idiolect' is an individual's personal version of their language
Gist of Idea
I can't distinguish elm trees, but I mean by 'elm' the same set of trees as everybody else
Source
Hilary Putnam (Meaning and Reference [1973], p.154)
Book Reference
'Meaning and Reference', ed/tr. Moore,A.W. [OUP 1993], p.154
A Reaction
This example is clearer and less open to hair-splitting than his water/XYZ example. You could, with Putnam, say that his meaning of 'elm' is outside his head, but you could also say that he doesn't understand the word very well.