Full Idea
I take 'coherence' to be a primitive, intuitive notion, not reduced to something formal, and so I do not venture a rigorous definition of it.
Gist of Idea
Coherence is a primitive, intuitive notion, not reduced to something formal
Source
Stewart Shapiro (Philosophy of Mathematics [1997], 4.8)
Book Reference
Shapiro,Stewart: 'Philosophy of Mathematics:structure and ontology' [OUP 1997], p.135
A Reaction
I agree strongly with this. Best to talk of 'the space of reasons', or some such. Rationality extends far beyond what can be formally defined. Coherence is the last court of appeal in rational thought.