The highest manifestation of life consists in this: that a being governs its own actions. A being that is always subject to the direction of another is somewhat of a dead thing.
Vita enim in hoc maxime manifestatur quod aliquid movet se ipsum; quod autem non potest moveri nisi ab alio, quasi mortuum esse videtur.
Original text (Latin)
Notes from Wikiquote
Chapter 14
Variants
Now slavery has a certain likeness to death, hence it is also called civil death. For life is most evident in a thing's moving itself, while what can only be moved by another, seems to be as if dead. But it is manifest that a slave is not moved by himself, but only at his master's command.