I've just been listening to a debate between Atheist Dan Barker and Catholic William Albrecht, which I found on a random podcast while searching for something to do with Dan Barker. The subject of the debate was "Did Jesus rise from the dead?"
Barker's basic case was that historical methods can never lead us to a conclusion that a miracle happened, as they only operate in the realms of probability and repeatability. In general, he gave a good, but hardly spectacular defence of this.
Albrecht, on the other hand set out to prove that not only was Jesus raised from the dead, but that he was raised in a physical body. The main body of the discussion in the debate ended up arguing about the meaning of the phrase 'spiritual body' as used by Paul in several places in his letters.
Albrecht seemed to think that if he could show that Paul meant something physical by using the phrase "spiritual body" that this would somehow prove the resurrection, and prove it was physical. Barker seemed to think that if he could show that Paul meant that Jesus was raised as a spirit (i.e. non physical) then this would prove that Paul's experience of Jesus was purely in the form of visions, which would not entail any kind of resurrection, physical or otherwise.
I felt that this whole aspect of the debate was utterly irrelevant. Because why should it matter what Paul meant by what he said in the letters?
How on earth could Paul possibly know anything about the nature of Christ's resurrection body?
He never knew Jesus pre-crucifixion, and he never met Jesus post-resurrection, except in the form of an appearance or vision. He did not touch Jesus.
So on what basis could Paul say anything at all, with authority, about the physical nature of the post-resurrection Christ? All he knew was what he has seen (but not touched), what others may have told him, what he deduced from the OT scriptures, and what he received by revelation or inspiration.
In a historical investigation of the question, we can't appeal to inspiration, revelation, interpretation of scripture, so the only authority Paul could have on this issue (the physical nature of Christ's resurrected body) would have to have come from others. Perhaps Cephas or James, both of whom Paul mentions speaking to in Jerusalem. Paul never says where he got this information.
It is debatable whether or not Paul actually meant a physical resurrection, but even if he did believe this, his authority is entirely second hand. Paul believed it because some unspecified person told him. That's not great authority.
And going further down this line of thought, how could many of the people who apparently saw (and may have touched) Jesus in the period between resurrection and ascension could definitely say that his flesh was "incorruptible"? Did anyone try to corrupt it? According to John, Thomas didn't even take Jesus up on the offer of touching him. Nobody could make this claim on experiential grounds. It can only be made on the basis of deduction, revelation, or inspiration.
So how did Paul know? He didn't. He asserted it because it fit with his theology, but Paul's opinion can tell us nothing about whether the resurrection was a historical event.