Ibn Daud may have been aware of the inadequacy of his argument from motion, and therefore he adds another, based upon the distinction between the "possible existent" and the "necessary existent"--a distinction and an argument due to Alfarabi and Avicenna. A possible existent is a thing whose existence depends upon another, and was preceded by non-existence. It may exist or not, depending upon its cause; hence the name _possible_ existent. A necessary existent is one whose existence is in itself and not derived from elsewhere. It is a necessary existent because its own essence cannot be thought without involving existence. Now the question is, Is there such a thing as a necessary existent, or are all existents merely possible? If all existents are possible, we have an infinite series, every link of which is dependent for its existence upon the link preceding it; and so long as there is no first there is nothing to explain the existence of any link in the chain. We must therefore assume a first, which is itself not again dependent upon a cause prior to it. This is by definition a necessary existent, which is the cause of the existence of everything else. This proof is compatible with God as a Creator. Having shown the existence and incorporeality of God we must now prove his unity. We shall base this proof upon the idea of the necessary existent. Such an existent cannot have in it any multiplicity; for if it has, its own essence would not be able to keep the elements together, and there would be need of an external agent to do this. But in this case the object would be dependent upon something else, which is incompatible with the idea of a necessary existent. Nor is it possible there should be two necessary existents; for the necessary existent, we have just shown, must be of the utmost simplicity, and hence cannot have any attribute added to its essence. Now if there is a second, there must be something by which the first differs from the second, or they are identical. Either the first or the second therefore would not be completely simple, and hence not a necessary existent.