 On the other hand, prima facie grounds for rehabilitating ordinary objects also have ancient roots in atomism, which holds that macrophysical objects are composed of particles which, while multiple, do meet the other criteria for true being laid down by Parmenides. On the assumption that composition follows regular principles, then, ordinary objects may be said to exist insofar as they are wholes composed of existing proper parts. This brings us face to face with what Peter van Inwagen (1990, 21 ff.) calls the Special Composition Question—under what conditions do unified wholes arise out of parts? Answering this question has turned out to be far from simple, and has latterly given rise to mereology, a special area of metaphysics devoted to investigating the principles of composition. Most of the other main arguments against the existence of artifacts and other ordinary objects reflect problems arising in the course of these investigations. For example, there is the material constitution problem. An artifact such as a cookie is made out of dough. So everywhere there is a cookie there is a coincident lump of (baked) dough, which shares all of its parts with the cookie. But this coincidence violates our intuitions about the identity of ordinary objects. Considerations of this sort have been used by van Inwagen (1990, 124 ff.) and others to argue that there are no artifacts. Cookie makers do not bring anything new into existence; they merely move pre-existing elementary things around. Thus, while debunking arguments show that we have no good reason to believe ordinary objects do exist, mereological problems show that we have good reasons to believe they do not.