DE MONARCHIA OF DANTE ALIGHIERI
CHAPTER II: God wills not that which is counter to the intention of nature.
1. As in the previous questions, so in the present one, we must assume some principle for informing the arguments which are to reveal the truth. For of what avail is it to labor even in speaking truth, if one have no basic principle?  And the principle is the sole root of the assumptions,  which are the mediums of proof.
2. Let us set up, then, this indisputable truth, that whatever is repugnant to the intention of nature is contrary to the will of God. If this were not true, its contrary would not be false, that whatever is repugnant to the intention of nature is not contrary to the will of God. And if this is not false, its consequences are not false. For in necessary consequences a false consequent is impossible without a false antecedent. 
3. But "not contrary to the will of" means one of two things, "to will" or "not to will;" just as "not to hate" means either to love" or "not to love; " for "not to love" does not mean "to hate," neither does "not to will" mean "to be contrary to the will of" as is self-evident. If these statements are not false, neither will it be false to assert that "God wills what He does not will," than which no greater fallacy exists.
4. I demonstrate as follows the verity of what has been said. That God wills an end for nature is manifest; otherwise the heavens would move to no purpose, which it is not possible to claim." If God should will an obstruction to this end, He would also will an end for the obstruction, or He would will to no purpose. Now the end of an obstruction is that the thing obstructed may exist no longer, so it follows that God wills the end of nature to exist no longer, when we have already said that He wills it to exist.
5. But if God did not will the obstruction to the end, it would follow from His not willing it that He cared nothing for the obstruction, whether it existed or not. Now he who cares nothing for the obstruction cares nothing for the end obstructed, and therefore has it not in his will, and what he has not in his will, he does not will. Hence if the end of nature can be impeded, and it can, it necessarily follows that God does not will an end of nature, and follows further, as before, that God wills what He does not will. That principle is therefore most true from the contradictory of which results such an absurdity. 
1. De Mon. 1. 2. 2; 2. 2. 1.
2. "Assumptions" are the major and minor premises.
3. Anal. Pr. 2. 2.
4. Dante proves this point De Mon. 1. 3. 2; 1. 10. 1; 2. 7. 2, 3; and 3. 15. 1. See also the quotations in the notes to these paragraphs. Dante expresses the idea most clearly, perhaps, in Par. 1. 109: "In that order which I say have all natures their propension, through divers lots, more or less near to their origin, whereby they move to divers ports through the sea of being, and each with instinct given to it to bear it."
5. Miss Hillard notes the use of proof by reduction to absurdity, Conv. 2. 9. 4.