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:
1.Sam is stealing money from work.
2.Losing money by theft causes harm to Sam's employers.
3.(One ought to not cause harm to his employers.)
4.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? (Philosophy-Index)
Why doesn't Hume's Guillotine effectively decapitate ethical intuitionism? Simply because ethical intuitionism holds that there are certain moral facts that are self-evident, and are not based on inference or conscious reasoning. One definition of Ethical Intuitionism is a view in moral epistemology according to which some moral truths can be known without inference ("Ethical Intuitionism"). Hume is arguing that one cannot move from a descriptive fact (how something is) to a prescriptive fact (how something ought to be) by means of inference or deduction. However, that is not what ethical intuitionists do. Listen to Brian Zamulinski:
Inference is an intellectual movement from proposition to proposition. Apprehension is the acquisition of a belief in response to a state of affairs. Our ability to apprehend states of affairs is not fundamentally an ability to make inferences, no matter what sorts of inferences. It is an ability to see that such and such is the case. With evolutionary intuitionism, we intuitively apprehend the fact, say, that torture is wrong. We do not infer the belief that torture is wrong from other propositions. Since inference is not involved, the impossibility of inferring an "ought" from an "is" is not relevant. The is/ought gap is of no significance whatsoever for evolutionary intuitionism (Evolutionary Intuitionism: A Theory of the Origin and Nature of Moral Facts , p. 112).