Full Idea
The 1st standard objection to coherence is the 'isolation problem', that contingent apparently-empirical beliefs might be justified in the absence of any informational input from the extra-conceptual world they attempt to describe.
Gist of Idea
Coherence seems to justify empirical beliefs about externals when there is no external input
Source
Laurence Bonjour (A Version of Internalist Foundationalism [2003], 3.2)
Book Reference
Bonjour,L/Sosa,E: 'Epistemic Justification' [Blackwells 2003], p.53
A Reaction
False beliefs can be well justified. In a perfect virtual reality we would believe our experiences precisely because they were so coherent. Messengers from the front line have top priority, but how do you detect infiltrators and liars?