This is a difficult book on a difficult subject. Gibbons's ostensive topic is the apparent incompatibility of three plausible ideas: that belief aims at truth, that you ought always to believe what it is reasonable for you to believe, and that there are cases of reasonable false belief. Gibbons articulates his puzzle (chapter 1) as a clash between two intuitive principles:
(T) You ought to believe that p only if it is true that p.
(J ) You ought to believe that p if and only if you are justified in believing that p.