From the categories he passes to a discussion of the most fundamental concepts in the Aristotelian philosophy, matter and form. And here his method of proving the existence of matter is Aristotelian and new. It is based upon the discussion in Aristotle's Physics, though not necessarily derived from there directly. Primary matter, he says, is free from all form. There must be such, for in the change of one thing to another, of water to air for example, it cannot be the _form_ of water that receives the form of air; for the form of water disappears, whereas that which receives the new form must be there. Reason therefore leads us to assume a common substrate of all things that are subject to change. This is primary matter, free from all form. This matter being at the basis of all change and becoming, could not itself have come to be through a similar process, or we should require another matter prior to it, and it would not be the prime matter we supposed it to be. This last argument led Aristotle to the concept of an eternal matter, the basis of becoming for all else besides, itself not subject to any such process. It is an ultimate, to ask for the origin of which would signify to misunderstand the meaning of origin. All things of the sublunar world originate in matter, hence matter itself is the unoriginated, the eternal. Ibn Daud as a Jew could not accept this solution, and so he cut the knot by saying that while it is true that matter cannot originate in the way in which the composite objects of the sublunar world come to be, it does not yet follow that it is absolutely ultimate and eternal. God alone is the ultimate and eternal; nothing else is. Matter is a relative ultimate; relative, that is, to the composite and changeable objects of our world; but it is itself an effect of God as the universal cause. God created it outright.