In Whitehead's view, then, concepts such as "quality", "matter", and "form" are problematic. These "classical" concepts fail to adequately account for change, and overlook the active and experiential nature of the most basic elements of the world. They are useful abstractions, but are not the world's basic building blocks. What is ordinarily conceived of as a single person, for instance, is philosophically described as a continuum of overlapping events. After all, people change all the time, if only because they have aged by another second and had some further experience. These occasions of experience are logically distinct, but are progressively connected in what Whitehead calls a "society" of events. By assuming that enduring objects are the most real and fundamental things in the universe, materialists have mistaken the abstract for the concrete (what Whitehead calls the "fallacy of misplaced concreteness").
 What basic concepts did Whitehead believe were not questionable?