From The Art and Popular Culture Encyclopedia
The fact-value distinction is a concept used to distinguish between arguments that can be claimed through reason alone and those in which rationality is limited to describing a collective opinion. In another formulation, it is the distinction between what is (can be discovered by science, philosophy or reason) and what ought to be (a judgment which can be agreed upon by consensus). The terms positive and normative represent another manner of expressing this, as do the terms descriptive and prescriptive, respectively. Positive statements make the implicit claim to facts (e.g. water molecules are made up of two hydrogen atoms and one oxygen atom), whereas normative statements make a claim to values or to norms (e.g. water ought to be protected from environmental pollution).