To Florus 3.120

To apply the example to our case, then: you could have understood what to call free from the definition of unfree. Even if your understanding grew dark when defining free will, so that you thought something could be called free while confined to one of two opposites, you should have noticed that captivity—that is, lack of freedom—can be explained only as being bound to one of two opposites. Freedom must therefore be named by denying captivity. Since what is unfree is forced to cling to one of the opposites, its contrary, what is free, cannot be bound to either.