Concerning the Nature of Good, Against the Manichaeans

But fire is eternal, not as God is eternal, because, though without end, yet is not without beginning; but God is also without beginning. Then, although it may be employed perpetually for the punishment of sinners, yet it is mutable nature. But that is true eternity which is true immortality, that is that highest immutability, which cannot be changed at all. For it is one thing not to suffer change, when change is possible, and another thing to be absolutely incapable of change. Therefore, just as man is called good, yet not as God, of whom it was said, "There is none good save God alone;" and just as the soul is called immortal, yet not as God, of whom it was said, "Who alone has immortality;" and just as a man is called wise, yet not as God, of whom it was said, "To God the only wise;" so fire is called eternal, yet not as God, whose alone is immortality itself and true eternity.