so far as is possible, believe all that is true and only what is true.
first of all, i urge all of us to try to conduct ourselves by that standard. and second, i propose that it is something like tautological, that it follows from our status as creatures that believe things. to believe something is to take it to be true. if you take it to be false, you do not believe it, by definition, correct? so that the goal of truth is intrinsic to the nature of belief. so that takes care of the 'only what is true'. as to the all: i think there's an important pleasure and usefulness in knowing as much as possible! we're going to have to acknolwedge our extreme constraints in this regard, of course: that itself is part of trying to believe what's true.