The foremost logical prerequisite of a rigid classification is that a mental item belong to no more than one category. Such either/or logic presupposes a digital mode of thinking, which, unlike its analog counterpart, does not tolerate any "gray" shadings among mental fields. "Digital thinking" has a staccato character somewhat evocative of the on/off nature of the conventional light switch or the convulsive progression of time on digital clocks, in marked contradistinction to the smooth, legato style of the dimmer or the traditional analog clock.