The proposition has... been laid down in the most general terms, "All evils are negations." Thus for man death is evil; death is his non-existence. Illness, poverty, and ignorance are evils for man; all these are privations of properties.... The destruction of other things is likewise nothing but the absence of their form. After these propositions, it must be admitted as a fact that it cannot be said of God that He directly creates evil, or He has the direct intention to produce evil; this is impossible. His works are all perfectly good. He only produces existence, and all existence is good; whilst evils are of a negative character, and cannot be acted upon.
Ch.10 - Guide for the Perplexed (c. 1190) - Part III