  The objects of each of these categories may be regarded as sets which are varying in some manner: in case (i) over a Boolean algebra; in case (ii) over a topological space; in case (iii) over (discrete) time. A topos may be conceived, then, as a universe of “variable” sets. The familiar category Set is the special limiting case of a topos in which the “variation” of the objects has been reduced to zero.