 A type of justification (say, via perception) is fallible if and only if it is possible to be justified in that way in holding a false belief. A type of justification is defeasible if and only if that justification could be overridden by further evidence that goes against the truth of the proposition or undercut by considerations that call into question whether there really is justification (say, poor lighting conditions that call into question whether vision provides evidence in those circumstances).