Five doctors went duck hunting one day. Included in the group were a general practicioner, a pediatrician, a psychiatrist, a surgeon and a pathologist. After a time, a bird flew overhead.
The GP reacted first. He raised his shotgun, but then hesitated. "I'm not quite sure it's a duck," he said, "I think I should get a second opinion." Of course by that time, the bird was long gone.
Another bird appeared in the sky thereafter. This time, the pediatrician drew a bead on it. He too, however, was unsure if it was really a duck in his sights and besides, it might have babies. "I'll have to do some more investigations," he muttered, as the creature made good its escape.
Next to spy a bird flying was the sharp-eyed 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 fourth fowl 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. "Go see if that was a duck, will you?"