Full Idea
Some people view an externalist approach to epistemology as the essence of philosophical naturalism.
Gist of Idea
Externalism may be the key idea in philosophical naturalism
Source
David Papineau (Philosophical Naturalism [1993], Intro)
Book Reference
Papineau,David: 'Philosophical Naturalism' [Blackwell 1993], p.1
A Reaction
I suspect philosophers avoid psychology and mental events, simply because they are elusive. Externalism is a theory about justification, and independent of naturalism as a metaphysic.