Descriptive propositions are statements about fact, and are theoretically verifiable by any competent observer as either true, false, or having a certain degree of probability.  Normative propositions are assertions of value: their truth or falsity may therefore legitimately vary for different individuals.  Some of the most serious fallacies in eithical (and we might add scientific) reasoning arise from confusing the two types. 

Neither psychoanalysis nor anthropology is able to answer the questions of ethics.  Values and facts are independent types of meaning, and their relationship should not be oversimplified.

Philip Wheelwright, A Critical Introduction to Ethics

Quoted in Lowell L. Bennion, Religion and the Pursuit of Truth (pg. 66)