The philosopher John Stuart Mill, for example, attempted to demonstrate his moral theory, known as utilitarianism, by arguing that, if the only thing that people desire is pleasure, then pleasure is the only thing that is desirable (Mill, 1879).
Many philosophers think that Mill is equivocating between two different meanings of desirable. One interpretation means “able to be desired,” which he uses in the premise. The other interpretation is “thing that is good or should be desired,” which he uses in the conclusion. His argument would therefore be invalid, based on a subtle shift in meaning.