If you want to be fully convinced of the abominations of slavery, go on a southern plantation, and call yourself a negro trader. Then there will be no concealment; and you will see and hear things that will seem to you impossible among human beings with immortal souls.
No planet is more earth-like than Earth itself, so if life really does pop up readily in earth-like conditions, then surely it should have arisen many times right here on our home planet? And how do we know it didn't? The truth is, nobody has looked.

