The reconciliation of reason and nature which McDowell attempts in Mind and World is of fundamental importance. Reasons are things which prescribe or prohibit certain courses of action or the adoption of certain attitudes. To the extent that one fails to heed such prescriptions, all things being equal, one is at fault. There is a tendency to look on this last claim with some suspicion: from where to these prescriptions gain their authority? and how do they gain their authority? McDowell's answer to the first question is 'the world' and his response to the second question is to shrug his shoulders. His answer to the first question, moreover, determines his response to the second: if reasons are given to us by the world then asking how they come to gain their prescriptive force is akin to asking how objects (or events, or facts, or what have you) gain their causal efficacy; it is simply a brute fact which neither admits of nor requires explanation.
If McDowell is right then we shouldn't be suspicious of reasons or rational phenomena at all. This is not to say that we can close the book on the theory of reasons all together of course. It is to say that we can continue theorising about reasons - in relation to epistemology, metaethics, and the philosophy of mind - without having the doubt about their veracity lodged in the back of our minds; stultifying the whole exercise.