Hume’s guillotine

Episode #4 of the course “Philosophical ideas that everyone should know”

In his Treatise of Human Nature, philosopher David Hume gives the classic formula for what is still a central question in the philosophy of morals: How do descriptive statements (an ‘is’ statement) so quickly turn into prescriptive statements (a ‘should’ statement)? Hume argues that these two things should not be connected so closely. That is, he argues that “is” statements cannot lead to the morally-related conclusions that are often derived from these statements.

For example, consider the following two “is” statements: 1) Sally is stealing from Paul 2) Paul is harmed by theft. Many automatically jump to the conclusion that Sally should not steal from Paul because Sally should not harm Paul. Hume argues, however, that this conclusion is not a logical outcome of these two statements. That is, he argues that humans insert the premise that Sally should not harm Paul, when, in fact, that premise does not exist except by some moral code.

The issue that Hume pointed out was due to two strong, conflicting beliefs. First, humans accept that we live in a world of objective facts. However, if we make ethical judgments, we are stating something true about the world, which would be true no matter how we felt about it. These beliefs seem to conflict with Hume’s law. Hume was aware of the weight of his discovery and was concerned that “all the vulgar systems of morality” would be destroyed. The unbridgeable chasm between fact and value that Hume exposes makes the status of ethical claims doubtful, and in this way serves as the foundation of moral philosophy.

Hume’s law (or Hume’s guillotine) is usually conflated with a similar but separate view introduced by philosopher G.E. Moore in Principia Ethica (1903). Moore claimed that earlier scholars committed what he termed the “naturalistic fallacy.” This idea involves confusing ethical and natural concepts; thus “good” may be mistaken to mean the same as “pleasurable.” Moore alleged, however, that one should still ask whether the pleasurable is also good.

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