Full Idea
No two classes have exactly the same members, but two different attributes may be attributes of exactly the same things. Classes are identical when their members are identical. ...On the other hand, attributes have no clear principle of individuation.
Gist of Idea
Because things can share attributes, we cannot individuate attributes clearly
Source
Willard Quine (On the Individuation of Attributes [1975], p.100)
Book Reference
Quine,Willard: 'Theories and Things' [Harvard 1981], p.100