The third proof is based upon the idea of necessary existence. There is no doubt that there are existing things, for example the things we perceive with our senses. Now either all things are incapable of decay, or all are subject to genesis and decay, or some are and some are not. The first is evidently untrue for we see things coming into, and passing out, of being. The second hypothesis is likewise untrue. For if all things are subject to genesis and decay, there is a possibility that at some time all things might cease to be and nothing should exist at all. But as the coming and going of individuals in the various species in the world has been going on from eternity, the possibility just spoken of must have been realized--a possibility that is never realized is not a possibility--and nothing existed at all at that moment. But in that case how could they ever have come into being, since there was nothing to bring them into being? And yet they do exist, as ourselves for example and everything else. There is only one alternative left, therefore, and that is that beside the great majority of things subject to genesis and decay, there is a being not subject to change, a necessary existent, and ultimately one that exists by virtue of its own necessity (19). Whatever is necessary _per se_ can have no cause for its existence (20) and can have no multiplicity in itself (21); hence it is neither a body nor a corporeal power (12). We can also prove easily that there cannot be two necessary existents _per se_. For in that case the element of necessary existence would be something added to the essence of each, and neither would then be necessary _per se_, but _per_ that element of necessary existence which is common to both. The last argument against dualism may also be formulated as follows. If there are two Gods, they must have something in common--that in virtue of which they are Gods--and something in which they differ, which makes them two and not one. If each of them has in addition to divinity a differential element, they are both composite, and neither is the first cause or the necessary existent (19). If one of them only has this differentia, then this one is composite and is not the first cause.