LOGIC AND THE AUTONOMY OF ETHICS1

In every system of morality I have hitherto met with, I have always remark'd 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 surpriz'd 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 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 observ'd and explain'd; and at the same time a reason should be given for what seems altogether inconceivable, how this new relation can be a deduction from others which are entirely different from it. David Hume, Treatise, III.i.3. 1