The “is-ought fallacy” is another recurring ‘folk philosophy’ phrase – meaning “you can’t derive an ‘ought’ from an ‘is’”, after Hume.
This is a very interesting one, and it is undeniably common — even the exact phrase “you can’t derive an ‘ought’ from an ‘is’” returns something like 48000 hits on Google, and when you start adding variations, the number explodes. The principle is sometimes called ‘Hume’s Guillotine’, a label that seems to go back to philosopher Max Black in the 1960s. Others call it ‘Hume’s Law’, the source of which I have not been able to trace, although it does seem to be both more recent and less useful, given that there are plenty of other things that have also been called ‘Hume’s Law’. As is often the case with things that reduce to a slogan, it seems to be used in very different ways. Here are some various formulations that often get thrown around when talking about the ‘Is-ought fallacy’ or ‘Hume’s Guillotine’:
You can’t derive an ‘ought’ from an ‘is’.
You can’t derive an imperative from an indicative.
You can’t derive value judgments from factual judgments.
You can’t derive normative claims from factual claims.
You can’t derive evaluative claims from non-evaluative claims.
But oughts, imperatives, and value judgments are all very different things. ‘Ought’ statements, for instance, are generally indicative statements. What adds to the confusion is that all of these, even if they are often true, seem to have obvious counterexamples, yet they are all treated as absolute statements. There are many intriguing puzzles here, and the question is sometimes even raised as to whether the use of the principle is self-defeating. As a friend of mine, James Chastek, once joked, “We can’t derive an ‘ought’ from an ‘is’; therefore we ought not to try.”
Perhaps we should go back and look at the source of this slogan, David Hume (1711-1776).
Read the rest of this post at the First Thoughts blog.