Normative economics concerns what ought to be, involving value judgement. It is not a science because it contains no testable implications—therefore has no explanatory power. On the other hand, positive economics studies what is. This implies the existence of some theories which contain propositions such as “if A then B”, with the logical consequence of “if not B then not A”. A positive science is regarded as empirical in nature if such propositions or implications are testable, which requires that both A and B are observable entities.