  Is knowledge definable as justified true belief ("JTB")? We argue that one
can legitimately answer positively or negatively, depending on whether or not
one's true belief is justified by what we call adequate reasons. To facilitate
our argument we introduce a simple propositional logic of reason-based belief,
and give an axiomatic characterization of the notion of adequacy for reasons.
We show that this logic is sufficiently flexible to accommodate various useful
features, including quantification over reasons. We use our framework to
contrast two notions of JTB: one internalist, the other externalist. We argue
that Gettier cases essentially challenge the internalist notion but not the
externalist one. Our approach commits us to a form of infallibilism about
knowledge, but it also leaves us with a puzzle, namely whether knowledge
involves the possession of only adequate reasons, or leaves room for some
inadequate reasons. We favor the latter position, which reflects a milder and
more realistic version of infallibilism.
