Four doctors went duck hunting one day: a GP, a psychiatrist, a surgeon and a pathologist. After a time, a bird came winging overhead. The first to react was the GP who raised his shotgun, but then hesitated. "I'm not quite sure it's a duck," he said, "I think that I will have to get a second opinion." But of course by that time, the bird was gone.
Next to spot a bird flying overhead was the psychiatrist. Shotgun shouldered, he was more certain of his intended prey's identity. "Now, I know it's a duck, but does it know it's a duck?" The fortunate bird disappeared while the fellow wrestled with this dilemma.
Finally, a third bird sped past and this time the surgeon's weapon pointed skywards. BOOM. The surgeon lowered his smoking gun and turned nonchalantly to the pathologist beside him and said. "Go see if that was a duck, will you?"