We take a proposition to be certain when we have no doubt about its truth. We may do this in error or unreasonably, but objectively a proposition is certain when such absence of doubt is justifiable. The sceptical tradition in philosophy denies that objective certainty is often possible, or ever possible, either for any proposition at all, or for any proposition from some suspect family (ethics, theory, memory, empirical judgement, etc.). A major sceptical weapon is the possibility of upsetting events that cast doubt back onto what were hitherto taken to be certainties. Others include reminders of the divergence of human opinion, and the fallible sources of our confidences (see aenesidemus). Foundationalist approaches to knowledge look for a basis of certainty, upon which the structure of our systems of belief is built. Others reject the metaphor, looking for mutual support and coherence, without foundations. See also epistemology.