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.
Latin
Ita ergo ut ad causam referamus exemplum, intellegere potuisti vel definitione non liberi, quid deberes liberum nuncupare. Nam etsi in liberi arbitrii definitione intellegentia caligasset, ut id dici liberum posse arbitrareris, quod inter duo contraria uni doceretur adhaerere, respicere debuisti exponi aliter non posse captivum, id est non liberum, quam ut inter duo contraria uni vindicaretur addictum, et ideo nomen libertati captivitatis negatione tribuendum, ut, quoniam id, quod liberum non erat, alteri de contrariis cogeretur adhaerere, illud adversum ei id est liberum neutro sineretur addici.