In every system of morality, which I have hitherto met with, I have remarked, that the author proceeds for some time in the ordinary way of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surprized to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is, however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it should be observed and explained; and at the same time that a reason should be given, for what seemed altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it.David Hume, A Treatise of Human Nature, 3.1.1.27
If I'm understanding the problem correctly, Hume is noting that we live in a world of descriptive statements. In other words, whenever we are talking about the world as such, we are describing it. However, when we begin to talk about what we should and should not do (i.e. ought and ought not), we make prescriptive statements which go beyond our knowledge of how the world actually is. It isn't exactly clear how it's possible to bridge this gap from the is to the ought.
My only guess would be that, when we make ought statements, we are making some sort of conditional statements based on descriptives. If we want to be ethical, then we should act this way. We want to be ethical. Therefore, we should act this way. How we should act might be described in light of certain desires or motives we have and the best ways to bring these about. But he may have a point and I might be completely fudging it. As I type this out I can't help but suspect that I'm still drawing on prescriptions rather than descriptions.
