Moral reasoning

My problem begins with the 'is' and the 'ought'. Most philosophers since Hume have considered it to be a fundamental fact about ethics that an 'ought' may not be deduced from an 'is'. Many philosophers have used this alleged fact to support the view that one cannot move from a fact to an evaluation of it, from the non-moral to the moral, from the descriptive to the prescriptive, and from theoretical to practical knowledge, without proceeding via a moral principle. But does Hume's canon support these claims? What does the point about the 'ought' and the 'is' amount to