October 19, 2010

The Is-Ought Gap

Hume discusses the problem in book III, part I, section I of his A Treatise of Human Nature (1739):

In every system of morality, which I have hitherto met with, I have always remark'd, that the author proceeds for some time in the ordinary ways of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when all 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 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 shou'd be observ'd and explain'd; and at the same time that 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.

Hume is simply pointing out that no observations of the state of the world (what is) can give us guidance on morality (what we ought to do). People make links but are never able to demonstrate the logical connections when challenged.

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