Full Idea
Locke tried to show how all ideas were derived from experience by examining cases, but it was an uphill struggle; difficult cases include the ideas of identity, equality, perfection, God, power and cause.
Gist of Idea
It is unclear how identity, equality, perfection, God, power and cause derive from experience
Source
report of John Locke (Essay Conc Human Understanding (2nd Ed) [1694]) by Jonathan Dancy - Intro to Contemporary Epistemology 14.2
Book Reference
Dancy,Jonathan: 'Introduction to Contemporary Epistemology' [Blackwell 1985], p.213