That's the question that causes numerous sleepless nights for New Testament scholars like Kar Yong and Tony. Many have attempted to answer this question with technical journal articles and comprehensive section in monographs on St. Paul. And none has given the most lucid, succinct, and nonetheless witty answer as the prominent Charles H. Dodd:P/S: I'm wondering why only Malaysian scholars (Anthony Loke, Kar Yong, Kam Weng, Tony, etc) maintain blogs?
"...when Paul met Peter (Galatians 1.18), "We may presume they did not spend all their time talking about the weather."
(C.H. Dodd, The Apostolic Preaching and its Developments [UK: Hodder & Stoughon, 1936], 26, as quoted in Anthony Thiselton, The Living Paul: An Introduction to the Apostle and his Thought [UK: SPCK, 2009], 21)