Epistemology, the study of knowledge. How do we know what we know?
From Crooked Timber citing Donald Rumsfeld:
Reports that say that something hasn't happened are always interesting to me, because as we know, there are known knowns; there are things we know we know. We also know there are known unknowns; that is to say we know there are some things we do not know. But there are also unknown unknowns - the ones we don't know we don't know
Sound confused? Confusing? Read it again. It won the annual "foot in mouth award", but as the Language Log points out it shouldn't have. It is a really important idea for understanding how decision makers might act.