To Florus 3.119

And here you call someone a Caelestian for believing that anyone is free to do good. You therefore call free what you insist can do nothing but evil. Find, if you can, another definition of something unfree, and claim this as free! Even if you had lost your senses so completely that you could not see freedom’s definition in the thing itself, ought you not to have understood freedom’s essence from its opposite?

Suppose you were similarly uncertain what eyesight meant and defined it as follows: Sight means having the eyes gouged out, or being unable to see anything through some obstruction when it is time to see. Suppose you thought this definition fitted sight, but then turned to explain its opposite, blindness. You could find no other explanation except that, in an animal whose nature includes eyesight, the eyes have been gouged out or blocked by thick fluid, taking away the ability to see. You would surely come to your senses and recognize that one definition cannot fit contrary things.

Then, if blindness could mean only the deprivation of sight in an animal’s eye when it should be able to see, the definition of sight would emerge by denying the conditions used to explain blindness. Sight would be the proper capacity to see at the appropriate time, with eyes neither gouged out nor blocked. If you stubbornly resisted after this was shown, you would achieve only this: listeners would either think you opposed your own conscience with shameful obstinacy, or, if you convinced them that you really thought this way, they would judge your mind’s eyes as thoroughly gouged out as those of the person you had defined as possessing sight.