Full Idea
Quine (in 1941) attacked 'Principia Mathematica' because the items in the range of higher-order variables (attributes etc) are intensional and thus do not have a clearly defined identity relation.
Gist of Idea
Quine says higher-order items are intensional, and lack a clearly defined identity relation
Source
report of Willard Quine (Whitehead and the Rise of Modern Logic [1941]) by Stewart Shapiro - Foundations without Foundationalism 1.3
Book Reference
Shapiro,Stewart: 'Foundations without Foundationalism' [OUP 1991], p.16