In this paper we deal with the problem of the knowledge under uncertain conditions. Since Plato definition in Teetetos, knowledge has been defined as a true belief and this has been thougt as a clear and distinct idea. So, if we have any kind of knowledge, it can´t be uncertain. We are trying to show how this poit of view changed since fuzzy sets theory has been pro-posed: it is now possible to define knowledge even in conditions of epistemic uncertainty. This is the point of view we try to explain here.
Keywords: epistemology, uncertainty, knowledge, fuzzy sets, ontology.