2 Corinthians 12:7-10
And lest I should be exalted above measure through 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.
Paul clearly identifies his thorn in the flesh as “the messenger of Satan” and he says the work of this messenger was “to buffet me”. The word messenger is Angelos which you would recognize as angel, in this case an angel of Satan. The word buffet means, to strike repeatedly, to fight against like waves pounding against a ship. This thorn was a demonic spirit who persecuted Paul and made sure that he had a hard time of it. Paul lists a few types of things here that this messenger of Satan brings against him. Elsewhere Paul also lists specific examples of this demonic harassment:
2 Corinthians 11:23-28 Are they ministers of Christ? (I speak as a fool) I more; in labours more abundant, in stripes above measure, in prisons more frequent, in deaths oft, Of the Jews five times received I forty stripes save one. Thrice was I beaten with rods, once was I stoned, thrice I suffered shipwreck, a night and a day I have been in the deep; In journeyings often, in perils of waters, in perils of robbers, perils by mine own countrymen, in perils by the heathen, in perils in the city, in perils in the wilderness, in perils in the sea, in perils among false brethren; In weariness and painfulness, in watchings often, in hunger and thirst, in fastings often, in cold and nakedness. Beside those things that are without, that which cometh upon me daily, the care of all the churches.
So according to the scriptures we do know exactly what Paul’s thorn in the flesh was for he himself tells us. A demonic spirit was assigned to him to continually harass and persecute him. It should be noted also that the reason this thorn came about was because of the abundance of revelations. So we should be careful in making applications to ourselves regarding a thorn in the flesh as various types of problems. The thorn is not a bunch of problems; the thorn is a messenger (demonic spirit) of Satan. Also in all of the New Testament only Paul mentions this and no one else, so it is characteristic only of Paul. Today those who are completely surrendered to our Lord Jesus and are actively involved in fulfilling His ministry can also expect to be attacked by the enemy of man (i.e. Satan and his messengers.) In order to hold to a speculative theory that Paul's thorn in the flesh was some sort of disease or eyesight problem, one would have to ignore the clear teaching of the scriptures. The fact is nowhere in scripture does Paul himself refer to any such thing. If he did have a disease or eyesight problem then surely he would have mentioned it in the list of afflictions given above, but he does not.