An Old Joke

When I heard this a while back it was introduced to me as "an old joke", so I will introduce it as the same. 

A first year philosophy major bursts into his professors office early one morning. His face unshaven, hair a mess, bags under his eyes and the same clothes on from the day before. He had the look of someone who had been up all night. In his hands he clutched a book by Descartes and with tears in his eyes said, "Professor! Do I exist!?" His professor looked at him with a cheeky grin and asked, "Who wants to know?"