Thomas Aquinas

“ Therefore, as the more perfect the fire is in strength, so much the more perfectly does it impress its own form, so also the more perfectly does it corrupt the contrary. Hence that evil and corruption befall air and water comes from the perfection of the fire: but this is accidental; because fire does not aim at the privation of the form of water, but at the bringing in of its own form, though by doing this it also accidentally causes the other. ”

“ Now, a good will does not produce a morally bad act, since it is from the good will itself that a moral act is judged to be good. Nevertheless the movement itself of an evil will is caused by the rational creature, which is good; and thus good is the cause of evil.
Reply to Objection 2: Good does not cause that evil which is contrary to itself, but some other evil: thus the goodness of the fire causes evil to the water, and man, good as to his nature, causes an act morally evil. ”

“ Increase in intensity is in proportion to the nature of a thing. And as the form is a perfection, so privation removes a perfection. Hence every form, perfection, and good is intensified by approach to the perfect term; but privation and evil by receding from that term. Hence a thing is not said to be evil and worse, by reason of access to the supreme evil, in the same way as it is said to be good and better, by reason of access to the supreme good.
Reply to Objection 4: No being is called evil by participation, but by privation of participation. ”

“ The sinking of a ship is attributed to the sailor as the cause, from the fact that he does not fulfil what the safety of the ship requires; but God does not fail in doing what is necessary for the safety of all. ”

“ But since all contraries agree in something common, it is necessary to search for one common cause for them above their own contrary proper causes; as above the contrary qualities of the elements exists the power of a heavenly body; and above all things that exist, no matter how, there exists one first principle of being, as was shown above (Q [2] , A [3] ) .
Reply to Objection 1: Contraries agree in one genus, and they also agree in the nature of being; and therefore, although they have contrary particular cause, nevertheless we must come at last to one first common cause. ”