 The mainstream of philosophical tradition as Aquinas knew it, running from Aristotle through the Greek commentators and into the Arabic tradition, held that the material world, and all the kinds of living things in it, had always existed. These earlier Aristotelians still thought there was a place for God, as the first mover, but they understood God to be the initial, remote source of motions that had always existed, rather than an eternal being who created the universe anew. Aquinas understood Christianity to be committed to the latter view. He holds that the changeable universe around us has existed for only a finite amount of time; that God, in contrast, has existed eternally and unchangingly; and that God freely chose to create the changeable universe from nothing (ex nihilo). To bring a thing into existence ex nihilo is the defining feature of  creation,  in the strict sense of that term, and only God can do it (ST 1a 45). Everything else in the natural world causes new things to come into existence by modifying what is here already. The proper terminology for these natural changes is generation and corruption, which refer to a substance’s coming into or going out of existence by natural means, and alteration, which refers to a substance’s undergoing accidental change: continuing to exist through change to its shape, color, and other non-essential properties.