1 Corinthians 15:5

And that he was seen of Cephas, then of the twelve (KJV)

Then: The words are a reminder. The other apostles, major leaders of the church, witnessed the risen Lord. Two eyewitnesses were sufficient for a court of law (Magness class notes). These particular eyewitnesses had worked closely with Christ before his death. If anyone could recognize the Lord, they could.

Now: We hear Paul's words and trust his witness. No one would suffer and go through all Paul did, but for the truth. Paul talked with Peter (Cephas) who told him all. That would count as "hearsay," but for the fact of Paul's own encounter with Christ. He could compare his own knowledge with Peter's. We do the same thing. We compare our own contacts with Christ with Peter's and Paul's. They validate our knowledge and extend it

Barton and Osborne (p.221) compiled a complete listing of all appearances listed in the New Testament. You can compare that list to Paul's list to see what he is doing.

Extra Source: Bruce B. Barton (Wheaton IL: Tyndale House, c1999) online at http://books.google.com/books?id=H-f2mmq2uHEC&dq=Bruce+and+Corinthians&printsec=frontcover&source=in&hl=en&ei=91EFS8fUEMPblAey6YyhDA&sa=X&oi=book_result&ct=result&resnum=11&ved=0CB4Q6AEwCg#v=onepage&q=&f=false