 According to the traditional conception, which is presupposed by all accounts of computation discussed above, physical computation requires a physical medium, or substrate. Computation is an aspect of the organization and behavior of a physical system; there is no software without hardware. Thus, according to the traditional conception, if the universe is a cellular automaton, the ultimate constituents of the universe are the physical cells of the cellular automaton. It is legitimate to ask what kind of physical entity such cells are and how they interact with one another so as to satisfy their cellular automata rules.