1 Corinthians 5:3
For I verily, as absent in body, but present in spirit, have judged already, as though I were present, [concerning] him that hath so done this deed,
Note 5 at 1Co 5:3: This statement about Paul being "absent in body, but present in spirit," and the similar statement in 1Co 5:4, has been very widely interpreted. However, it seems very inconsistent with the rest of Scripture to believe that this was meant in a literal sense.
In 1Th 2:17, Paul said that he was physically absent from the Thessalonians but that his heart was with them. This, of course, does not mean that his physical organ had traveled in space or had the ability to be in more than one place at a time. He was simply expressing that he loved the Thessalonians so much that his thoughts and attention was still upon them.
Likewise here, Paul was simply expressing the intensity of his effort in resolving this problem involving incest. He had given it his attention, and they could be assured that he was with them in his prayers and interest.