This argument exhibits a well-worn pattern, and that alone ought to give us pause:A depends on B, but that dependence must depend on C, which dependence must in turn depend on D ... until we must come -for the regress cannot be infinite if it is to exist at all-to a prime mover, or to an ultimate fact-independent principle. However, I do not propose to dismiss Cohen's argument by means of guilt-by-association. Nor shall I plead that the regress can go on to infinity or resort to vague notions about justification needing no terminus since it is always a "holistic" affair of how a set of beliefs "cohere" together. My basis for rejecting this argument is a lot simpler.
First, consider what a principle is, in the sense under discussion. Principles are general rules of thought and action, asserting that certain lines of conduct are what we have reason to adopt in the sorts of circumstances they stipulate. Principles refer to what we may call standing reasons for how to think or act: the principle that one ought to help anyone in distress is tantamount to the proposition that one has as a rule good reason to do so.