 Van Inwagen’s main argument is that necessary and sufficient (particularly sufficient) conditions cannot be given for the kind of cohesion of parts that is supposed to bind atoms into complexes. None of the standard candidates—contact, fastening, adhesion, fusion, nor any acceptable disjunction of them—will suffice. They all have counterexamples. So the notion cannot be well defined. The arguments are subtle and interesting. The question might be raised whether van Inwagen gives sufficient consideration to the possibility of taking a line similar to that taken by Putnam on natural kinds. One fixes putative paradigm cases of non-living complexes, and says that the kind of cohesion required is whatever sort of bonding holds in those cases. It seems unlikely that there will not be a reasonably tidy, scientific account of how atoms bond. Nevertheless, van Inwagen’s mereological nihilism has a stopping point, namely that he believes that he, though a composite material object, really does exist. So organisms do exist.