Full Idea
The important thing about proper names is that it would be ridiculous to think that having linguistic competence can be equated in their case with knowledge of a necessary and sufficient condition.
Gist of Idea
Using proper names properly doesn't involve necessary and sufficient conditions
Source
Hilary Putnam (Explanation and Reference [1973], II B)
Book Reference
Putnam,Hilary: 'Mind Language and Reality: Papers vol 2' [CUP 1975], p.204