Philo Judaeus De Posteritate Caini 49

And it may become us next to consider on what ¹ account this same man is represented as founding and building a city, for it is only a multitude of men who have need of a city to dwell in; but the three who were the only human beings in existence at that time might have thought the foot of a mountain, or a small cave, a most sufficient abode. And I said, indeed, the three; but in all probability I might have spoken of him by himself; for the parents of Abel, who had been so treacherously slain, would never have endured to inhabit the same city with his murderer—a man who had committed fratricide, which is a greater pollution than even homicide.