 Moreover, McTaggart grants that the data provided by sense-perception are prima facie true. That is, if we seem to perceive P, then unless there are compelling a priori reasons or reasons deriving from other perceptions to believe ~P, we should believe P. This principle does real work in McTaggart’s system. We apparently perceive that objects are ordered in time: some events occur before others, whilst others are simultaneous. McTaggart holds that there is a powerful a priori argument that nothing is actually in time. (McTaggart’s argument will be discussed in section 6 of this article.) But there is no powerful argument that the objects that are apparently ordered by temporal relations are not ordered by some other (non-temporal) relation. Perception teaches us both that objects are in time and that they are ordered by some relation. Only the former is called into question by McTaggart’s argument against the reality of time. In this way, McTaggart arrives at the question: what is the nature of the ordering relation that, in conjunction with other facts, gives rise to the appearance that the objects which it orders are in time?