Screenshot from page 10 of Robert Rosen's Anticipatory Systems. Some text, especially the words "reactive paradigm" and "parsimony", are highlighted. The other answer lies in the fact that the reactive paradigm is universal, in the following important sense. Given any mode of system behavior which can be described sufficiently accurately, regardless of the manner in which it is generated, there is a purely reactive system which exhibits precisely this behavior. In other words, any system behavior can be simulated by a purely reactive system. It thus might appear that this universality makes the reactive paradigm completely adequate for all scientific explanations, but this does not follow, and in fact is not the case. For instance, the Ptolemaic epicycles are also universal, in the sense that any planetary trajectory can be represented in terms of a sufficiently extensive family of them. The reason that the Copernican scheme was considered superior to the Ptolemaic lies not in the existence of trajectories which cannot be represented by the epicycles, but arises entirely from considerations of parsimony, as embodied for instance in Occam's Razor. The universality of the epicycles is regarded as an extraneous mathematical artifact irrelevant to the underlying physical situation, and it is for this reason that a representation of trajectories in terms of them can only be regarded as a simulation, and not as an explanation.