I would like to recommend WD Ross's essay 'What Makes Right Acts Right'.
Ross is spot on in explaining what we mean by moral principles.
Ross argues that the principles of morality are in some way similar to the truths of nature (such as the universal law of gravitation). The law of gravity says that the force of gravity acts on all bodies universally - it is an absolute truth (excluding the quantum scale) but the actual movement of any given body depends on all of the forces acting on it.
Moral truths differ from universal laws of nature in that there is no causal relationship, instead there is a prima facie relationship which is self-evident, although this self-evidence does not appear from the beginning of our lives, it develops as we mature.
I particularly like the section where Ross says "The moral order expressed in these propositions is just as much part of the fundamental nature of the universe (and, we may add, of any possible universe in which there are moral agents) as is the spacial or numerical structure expressed in the axioms of geometry or arithmetic. In our confidence that these propositions are true there is involved the same trust in our reason that is involved in our confidence in mathematics."
Ross goes on to make a very valid point which I think gets to the nub of where Dancy is wrong:
"Our judgement about our actual duty in concrete situations have none of the certainty that attaches to our recognition of the general principles of duty".