Full Idea
Externalist accounts of non-inferential knowledge say what makes a true non-inferential belief a case of knowledge is some natural relation which holds between the belief state and the situation which makes the belief true.
Gist of Idea
Externalism says knowledge involves a natural relation between the belief state and what makes it true
Source
David M. Armstrong (Belief Truth and Knowledge [1973], 11.III.6)
Book Reference
Armstrong,D.M.: 'Belief Truth and Knowledge' [CUP 1981], p.157
A Reaction
Armstrong's concept is presumably a response to Quine's desire to 'naturalise epistemology'. Bad move, I suspect. It probably reduces knowledge to mere true belief, and hence a redundant concept.