Full Idea
A set of unrelated beliefs seems less coherent than a tightly organized conceptual scheme that contains explanatory principles that make sense of most of your beliefs; this is why inference to the best explanation is an attractive pattern of inference.
Gist of Idea
A coherent conceptual scheme contains best explanations of most of your beliefs
Source
Gilbert Harman (Rationality [1995], 1.5.2)
Book Reference
Harman,Gilbert: 'Reasoning Meaning and Mind' [OUP 1999], p.33
A Reaction
I find this a very appealing proposal. The central aim of rational thought seems to me to be best explanation, and I increasingly think that most of my beliefs rest on their apparent coherence, rather than their foundations.