Full Idea
There is as yet no substantial theory of inference or reasoning. To be sure, logic is well developed; but logic is not a theory of inference or reasoning. Logic is a theory of implication and inconsistency.
Gist of Idea
We have a theory of logic (implication and inconsistency), but not of inference or reasoning
Source
Gilbert Harman ((Nonsolipsistic) Conceptual Role Semantics [1987], 12.2.2)
Book Reference
Harman,Gilbert: 'Reasoning Meaning and Mind' [OUP 1999], p.212
A Reaction
One problem is that animals can draw inferences without the use of language, and I presume we do so all the time, so it is hard to see how to formalise such an activity.