Whoever produces all that is real in a thing is its cause.
God produces all that is real in sin.
Therefore God is the cause of sin.
answer
I might content myself with denying the major or the minor, because the term 'real' admits of interpretations capable of rendering these propositions false. But in order to give a better explanation I will make a distinction. 'Real' either signifies that which is positive only, or else it includes also privative beings: in the first case, I deny the major and I admit the minor; in the second case, I do the opposite. I might have confined myself to that; but I was willing to go further, in order to account for this distinction. I have therefore been well pleased to point out that every purely positive or absolute reality is a perfection, and that every imperfection comes from limitation, that is, from the privative: for to limit is to withhold extension, or the more beyond. Now God is the cause of all perfections, and consequently of all realities, when they are regarded as purely positive. But limitations or privations result from the original imperfection of creatures which restricts their receptivity. It is as with a laden boat, which the river carries along more slowly or less slowly in proportion to the weight that it bears: thus the speed comes from the river, but the retardation which restricts this speed comes from the load. Also I have shown in the present work how the creature, in causing sin, is a deficient cause; how errors and evil inclinations spring from privation; and how privation is efficacious accidentally. And I have justified the opinion of St. Augustine (lib. I, Ad. Simpl., qu. 2) who explains (for example) how God hardens the soul, not in giving it something evil, but because the effect of the good he imprints is restricted by the resistance of the soul, and by the circumstances contributing to this resistance, so that he does not give it all the good that would overcome its evil. 'Nec (inquit) ab illo erogatur aliquid quo homo fit deterior, sed tantum quo fit melior non erogatur.' But if God had willed to do more here he must needs have produced either fresh natures in his creatures or fresh miracles to change their natures, and this the best plan did not allow. It is just as if the current of the river must needs be more rapid than its slope permits or the boats themselves
be less laden, if they had to be impelled at a greater speed. So the limitation or original imperfection of creatures brings it about that even the best plan of the universe cannot admit more good, and cannot be exempted from certain evils, these, however, being only of such a kind as may tend towards a greater good. There are some disorders in the parts which wonderfully enhance the beauty of the whole, just as certain dissonances, appropriately used, render harmony more beautiful. But that depends upon the answer which I have already given to the first objection.
objection vi
Whoever punishes those who have done as well as it was in their power to do is unjust.
God does so.
Therefore, etc.