In Confessions, book 7 chapter XII, St. Augustine writes:
- If things are sovereignly good, they are incorruptible.
- If a thing is corruptable, it contains good.
- Evil is either an incorruptible substance or a corruptible substance.
- If evil in incorruptible, it's sovereignly good.
- If evil is corruptable, it contains good.
To my mind, in derivations 4 and 5 Augustine perceives implication in reverse order. That violates the laws of logic. Am I right?
a) $A → ¬B (if sovereignly good, they were incorruptible)
b) supposing ¬B, therefore ¬B → A (for either it should be an incorruptible substance, and so a chief good)
Here is a logical mistake, like: A → ¬B (if color is black, it's not green); ¬B → A (color is not green, therefore, it's black)