If we can infer from the conceivability to the possibility of such creatures, then the falsity of constitutive micropsychism seems to follow. For the constitutive micropsychist holds that the facts about micro-subjects wholly account for the existence of macro-level conscious subjects. But if it is possible for the facts about micro-subjects to obtain in the absence of any macro-level consciousness—which on the face of it seems to be the case in a world of micro-experiential zombies—then it seems that the facts about micro-subjects cannot wholly account for the existence of macro-level consciousness (it’s generally assumed that for fact X to ground fact Y, it must be the case that X necessitates Y). In that sense, it’s impossible for subjects to sum.