Thomas Aquinas — on Phlm 1:10 (Commentary on Philemon)

“Having expressed his confidence in Philemon's goodness, here he states his request. And first he indicates the person on whose behalf he pleads; second, he concludes the request. The first is subdivided into two, because first by describing the person he shows that he has received him in spiritual birth; second, by a change of status. Therefore he says, I plead with you for my own son, whom I have begotten in prison, for Onesimus, who is his present concern. And acquiring a son in default of time, he loves him more, as an old man loves sons born to him in his old age. Genesis 37:3: 'Now Israel loved Joseph above all his sons, because he had him in his old age.' This one [Onesimus] was given birth in chains.”