The standard objections to a coherence theory of empirical justification: notes from Toward a Coherence Theory of Empirical Knowledge by BonJour
The Alternative Coherent Systems Objection
The system of beliefs which constitutes empirical knowledge is epistemically justified solely by virtue of its internal coherence. This however leaves out the wider view that there may be other, probably infinitely more, systems of belief which are different and incompatible yet equally coherent. This leads to the inevitable conclusion that we have no way of knowing that the beliefs we actually hold are true, or rather that any other arbitrarily chosen system of belief is untrue or less true (dependent of course on the degree of coherence of each belief system in question). This does not amount to relativism as what is being scrutinized is a system of beliefs which must at the minimum have a high degree of coherence.
The Input Objection
A system of beliefs, which has a perfectly coherent internal structure, might in fact be completely disconnected from the real world (the world that it purports to describe – just think politics).
“Nothing about any requirement of coherence dictates that a coherent system of beliefs need receive any sort of input from the world or be in any way causally influenced by the world.” (p108)
This undermines the systems claim to empirical knowledge. This objection runs deep, just think “what would Heidegger say”. It questions the very ability of consciousness to relate a system of beliefs to ‘things-in-the-world’ or ‘things-in-themselves’.
The Problem of Truth
This objection strikes at the epistemological theories cognitive goal of truth. This objection, and appeal to a coherence theory of truth was made by absolute idealists. In effect, philosophers construed truth as simply identical with justification-in-the-long-run. The problem is that this conception of truth is motivated by a pre-conception of justification. Such a conception of truth must be independently motivated, and therefore not necessarily identical. This means that the problem of bridging the gap between justification and truth remains unsolved.