Slavery might have been legal but that did not make it just. From the first paragraphs of Thoughts upon Slavery.
Slavery imports* an obligation of perpetual service, an obligation which only the consent of the master can dissolve… It generally gives the master an arbitrary power of any correction, not affecting life or limb. Sometimes even these are exposed to his will, or protected only by a fine, or some slight punishment, too inconsiderable to restrain a master of a harsh temper. It creates an incapacity of acquiring anything, except for the master’s benefit. It allows the master to alienate+ the same, in the same manner as his cows and horses. Lastly, it descends in its full extent from parent to child, even to the last generation.
The beginning of this may be dated from the remotest period of which we have an account in history. It commenced in the barbarous state of society, and in process of time spread into all nations. It prevailed particularly among the Jews, the Greeks, the Romans, and the ancient Germans; and was transmitted by them to the various kingdoms and states which arose out of the Roman Empire. But after Christianity prevailed, it gradually fell into decline in almost all parts of Europe. This great change began in Spain, about the end of the eighth century; and was become general in most other kingdoms of Europe, before the middle of the fourteenth.
- *i.e. implies.
- + legally confiscate unto himself, so that the slave may own nothing.