Hume's Guillotine, also known as the is-ought problem or Hume's law is a criticism of writings by ethicists who make normative claims (about what ought to be) based on positive premises (about what is). The problem was articulated by David Hume in his most important philosophical work, A Treatise of Human Nature (Book III, §I).

Hume argued that one cannot make a normative claim based on facts about the world, implying that normative claims cannot be the conclusions of reason.

The term "Hume's Guillotine" is meant to describe the severance of "is" statements from "ought" statements, which similarly, and colourfully, illustrates the resulting removal of the head from many ethical arguments.

One may consider the following moral argument as an example of an is-ought problem:

Sam is stealing money from work.

Losing money by theft causes harm to Sam's employers.

(One ought to not cause harm to his employers.)

Therefore, Sam ought to stop stealing money from work.

Premises 1 and 2 are "is" statements, describing facts of what is happening. Premise 3 and Conclusion 4 are "ought" statements, that describes how things should be happening. But what is the source of this knowledge? This argument appears to be valid if the premises are true, but unless we can logically support Premise 3, it is not sound. What can possibly give us rational knowledge that things ought to be different than the way things are?

Hume argues that we cannot, and that ought statements, and other supposed moral knowledge, are not rational.

Opponents

Not all philosophers agree that Hume's Guillotine is a real problem, or that it is correctly described.

Some philosophers believe that certain types of "is" statements may imply or infer "ought" statements. For example, any statement that corresponds to a human goal or value, especially (or exclusively) one connected to human behaviour, suggests that certain "ought" statements have validity. For example, the statement "Sam ought to stop stealing money from work if he wants to avoid punishment" appears to be, on its own, a correct derivation of an "ought" from the "is" statements mentioned above. Similarly, other arguments in the form of "A ought to B to achieve goal C", such as "Jill ought to finish her philosophy paper to earn credits for her course", do not present a problem. Opponents to this idea, however, point out that the "ought" in these cases is not derived from the "is" facts but from the goal, which is, itself, an "ought".

John Searle presented another situation which may represent the correct formation of an "ought" from an "is". Searle held that the making of a promise, by definition, creates an obligation for the promise-maker, which, as an "is", implies an "ought". Some opponents to Searle's claim, however, say that this still creates an unfounded premise, that one ought to keep one's promises.

Others seek a less logical source for "oughts" as self-evident or natural facts that can be known, as they exist in humans as common ideas, or ideals, as a product of biological evolution (or other sources).