  This means that considering whether or not P requires that one consider both sides of the question whether P, i.e., that one considers arguments in favour of P and arguments in favour of not-P (or, more generally, of something incompatible with P; see PH I 10). It would not be enough, for instance, to look at one argument in favour of P and then reject that argument as invalid; while doing so would indeed leave one neither believing P nor not-P (assuming one had no further evidence either way), one would not have considered both sides of the question, since one would not yet have considered any arguments in favour of not-P. (This condition will be important later when we consider the Modes of Skepticism—see below, 3.5.2.)