Book 3 · Section 50

(50) Why does he order all slaves to be circumcised, those born in the family and also those who are bought? (#Ge 17:12). The literal meaning is plain; for it is fitting that servants should imitate their masters on account of their necessary employment, and the services to which they are bound in life. But with respect to the inner object of the command, those dispositions are what may be called born in the family which are influenced by nature itself, and those are bought which can be changed for the better by teaching and instruction. Each of these has its appropriate employment, and requires like a plant to be cleared and pruned in order that the good and fruitful parts may acquire constancy; for fertile plants produce many superfluous things by reason of their fecundity, and those superfluities must be cut away; but those who are taught by instructors cut away their ignorance.