What did Paul mean by thorn in the flesh?

Paul, in one of his Epistles, speaks of what he calls "A thorn in the flesh," and the phrase has led to much guessing and discussion. Do you have any idea of what he meant by the expression?

I know well what he means by the "thorn in the flesh." Though not a tall man, he was what you might call a strong man, without any symptom of bodily deformity or weakness. But it was his former course of bitter and vindictive opposition to Jesus, to the truth, that formed the thorn in his flesh, for amid all his labours and triumphs as an apostle, his former attitude to the cause he loved so much was ever coming up before his mind as a matter for bitter regret.

Some imagine the "thorn" to have been a weakness in the eyes because he speaks of employing a friend to write for him. Paul had anything but weak eyes. It was all the other way. His eyes were strong and piercing, eagle-like in their glance.

And you need not wonder at such as Paul employing another to write for him, for besides his continual labours in speaking, he was a very eloquent public speaker—he was constantly sending letters of advice to his converts in various places, and as many of these were dictated under inspiration, it became necessary to employ one who could take down the words quickly as they were uttered.

Hafed, Prince of Persia—His Experiences in Earth Life and Spirit Life