Is–ought problem
Etymology
Identified by David Hume in "A Treatise of Human Nature" (1739), who noted writers switching imperceptibly from "is" to "ought." It is also nicknamed "Hume’s guillotine," a term coined by Max Black, for the way it cleanly severs descriptive premises from prescriptive conclusions.
noun phrase · /IZ-AWT PROB-lum/
Hume’s observation that you cannot logically derive a claim about what ought to be from claims about what merely is.
What it names
A handle for spotting the silent leap from facts to values in an argument — reach for it when someone slides from “this is natural” to “this is right.”
Example
Arguing that because humans evolved to eat meat we ought to eat meat runs straight into the is–ought problem.
Related idea
Close kin to the naturalistic fallacy, but not identical: Hume’s point is about logical derivation, while the naturalistic fallacy concerns defining “good” in terms of natural properties.