Philo Judaeus Quod Omnis Probus Liber Sit 23

But we must consider that not only is the man who feels no anxiety to avoid death incapable of being made a slave, but the same privilege belongs to those who are indifferent to poverty, and want of reputation, and pain, and all those other things which the generality of men look upon as evils, being themselves but evil judges of things, since they pronounce a man a slave from a computation of what things he has need of, looking at the duties which he is compelled to perform, when they ought to look rather at his free and indomitable disposition;