Under what conditions do we have inferential knowledge? I propose and defend the
following principle: S knows that p via inference only if S knows all the premises
essentially involved in her inference in support of p - "KFK" for short. Even
though KFK is at least tacitly endorsed by many figures in the history of
philosophy, from Aristotle through Descartes, and Kant to Bertrand Russell -and,
more recently, by David Armstrong - KFK has fallen into disfavor among
epistemologists over the past fifty years. In response to Edmund Gettier's legendary
paper, many have proposed views according to which one's reasoning is a
source of knowledge even if one fails to know some or all premises essentially
involved in one's reasoning, while others have given up offering a theory of inferential
knowledge and have focused on reasoning as a source of justified belief
instead. Unfortunately, these accounts that deal with inferential knowledge are
problematic; they cannot, for example, fully explain our common practice of evaluating
negatively inferences with unknown premises.