The is–ought problem, as articulated by David Hume, arises when one makes claims about what ought to be based solely on statements about what is. ... This is one of the biggest epistemological questions.

David Sloan Wilson (recalling a conversation) proposes to insert a third word: "want": "If I am thirsty, I want water from a well so I ought to go there." Wanting something is how one moves from a fact to a value statement.

It also means that rationality kicks in after desire.