What is the ‘is-ought’ fallacy?

The ‘is-ought’ fallacy, associated most notably with David Hume, is a fallacy committed in reasoning from descriptive premises to normative or prescriptive conclusions. Descriptive statements describe things: they tell us how the world is (and as such typically contain an ‘is’). However, normative or prescriptive statements tell us how the world ought to be (and as such typically contain as ‘ought’). Reflect for a moment on this argument: (1) either that flying object is a bird or it is a plane; (2) that flying object is not a bird; therefore, (3) that flying object is a plane. This argument is what logicians call valid because the premises entail the conclusion. In other words, so long as the premises are true it’s impossible for the conclusion to be false. Compare this kind of argument: (1) Evolution is a competition for survival; (2) we humans are the products of evolution; therefore, (3) we humans ought to compete to survive. Is this argument valid? It may appear to have some initial plausibility, but it doesn’t seem at all obvious that it is. From the fact that evolution is a competition for survival and the fact that humans are the result of evolution, it doesn’t follow that humans ought to compete to survive. All that really follows is that so far we have - successfully - competed for survival. It’s important to note that that’s not to say that humans ought not to compete to survive, only that these facts alone do not establish any conclusion about what ought to happen whatsoever. This kind of fallacy is often committed by so-called Social Darwinists, who believe that evolutionary theory has implications for how we ought to arrange society or live our lives.

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