A Thorn in the Flesh
There is much speculation about what the thorn in Paul's flesh was.
And lest I should be exalted above measure through the abundance of the revelations, there was given to me a thorn in the flesh, the messenger of Satan to buffet me, lest I should be exalted above measure. For this thing I besought the Lord thrice, that it might depart from me. And he said unto me, My grace is sufficient for thee: for my strength is made perfect in weakness. Most gladly therefore will I rather glory in my infirmities, that the power of Christ may rest upon me. Therefore I take pleasure in infirmities, in reproaches, in necessities, in persecutions, in distresses for Christ's sake: for when I am weak, then am I strong.
- 2 Corinthians 12:7-10 (KJV)
I think that the thorn in the flesh was meant as a permanent reminder for Paul about where he had come from (persecuting Christians).
Maybe Paul's conversion can tell us more.
And Saul, yet breathing out threatenings and slaughter against the disciples of the Lord, went unto the high priest, And desired of him letters to Damascus to the synagogues, that if he found any of this way, whether they were men or women, he might bring them bound unto Jerusalem. And as he journeyed, he came near Damascus: and suddenly there shined round about him a light from heaven: And he fell to the earth, and heard a voice saying unto him, Saul, Saul, why persecutest thou me? And he said, Who art thou, Lord? And the Lord said, I am Jesus whom thou persecutest: it is hard for thee to kick against the pricks. And he trembling and astonished said, Lord, what wilt thou have me to do? And the Lord said unto him, Arise, and go into the city, and it shall be told thee what thou must do. And the men which journeyed with him stood speechless, hearing a voice, but seeing no man. And Saul arose from the earth; and when his eyes were opened, he saw no man: but they led him by the hand, and brought him into Damascus. And he was three days without sight, and neither did eat nor drink.
- Acts 9:1-9 (KJV)
It is highly likely that Paul was (somewhat permanently) blinded by the light from heaven. That would explain why he was unable to see for three days. Some have suggested that it's a bit like looking into the sun, and that that would permanently damage the eye. If that's true, Paul was probably not able to see the things in front of them properly, and would have had to rely on peripheral vision.
The comment about scales falling of Paul's eyes is in agreement with that kind of "looking into the sun" damage to the eyes.
For I will shew him how great things he must suffer for my name's sake. And Ananias went his way, and entered into the house; and putting his hands on him said, Brother Saul, the Lord, even Jesus, that appeared unto thee in the way as thou camest, hath sent me, that thou mightest receive thy sight, and be filled with the Holy Ghost. And immediately there fell from his eyes as it had been scales: and he received sight forthwith, and arose, and was baptized.
- Acts 9:16-18 (KJV)
It would also explain why he wrote with large letters.
Ye see how large a letter I have written unto you with mine own hand.
- Galatians 6:11 (KJV)
I think God left the thorn there as a reminder for Paul where he had come from. That kind of daily reminder draws us closer to God, and leads us to a better understanding, and appreciation of God's Mercy.
Paul is strong in his weakness. He can't boast about anything, so no pride, etc. It kept him humble.
God did him a favor really.