Full Idea
Nozick says Gettier cases aren't knowledge because the proposition would be believed even if false. Proper justification must be more sensitive to the truth ("track the truth").
Gist of Idea
A true belief isn't knowledge if it would be believed even if false. It should 'track the truth'
Source
report of Robert Nozick (Philosophical Explanations [1981], 3.1) by Jonathan Dancy - Intro to Contemporary Epistemology 3.1
Book Reference
Dancy,Jonathan: 'Introduction to Contemporary Epistemology' [Blackwell 1985], p.37
A Reaction
This is a bad idea. I see a genuine tree in my garden and believe it is there, so I know it. That I might have believed it if I was in virtually reality, or observing a mirror, won't alter that.