Full Idea
Davidson observes that the inference from a thought being identified by a relation to something outside the head does not entail that the thought is not wholly in the head, just as sunburn is identified by external factors, but is still in the skin.
Gist of Idea
External identification doesn't mean external location, as with sunburn
Source
report of Donald Davidson (Knowing One's Own Mind [1987]) by Mark Rowlands - Externalism Ch.8
Book Reference
Rowlands,Mark: 'Externalism' [Acumen 2003], p.144
A Reaction
Rowlands (an externalist) agrees, and this strikes me as correct, and it needs to be one of the fixed points in any assessment of externalism.