What's your favorite philosophy joke?

Question:

What's your favorite philosophy joke?

A 90 year old man goes to see his doctor and says "Doc! My 18 year old wife is pregnant!"
The doctor says "let me tell you a story. A blind man went hunting and took an umbrella instead of a gun. When he found the bear, the blind man raised his umbrella, pointed it directly at the bear and shot it dead."
"IMPOSSSIBLE," says the patient, "someone else must have shot the bear!"
"Exactly," says the doctor.

I thought that was hysterical. Anyone else want to share philosophical jokes? (no Zen hot dogs with everything, please!)

Best Answer - Chosen by Asker: Ontology joke:

"Knock Knock"

"Who's there?"

<end of joke>