Intensity and remission are said in respect to some term. But something is found worse than another. Therefore, there must be found something which is the worst, in which there is the term of evil things. And this must be the principle of all evil things, just as the highest good is the principle of good things. But from a good, which has the nature of light, there cannot be the creation of darkness.
Every agent acts from the presupposition of the first agent.
But free will, when it sins, does not act from the presupposition of divine action. For there is some sin, such as fornication and adultery, which cannot be separated from their deformity, and which cannot be from God. Therefore, it is necessary that free will be a first agent or that it be reduced to some other first agent than God. Hence, from the consideration of contraries, which with matter are given as the principles in nature, they came to the conclusion that the first principles consisted in two things. They did so because their consideration of contraries was flawed in three ways.