Here's a description I like: you know more about your own body than your doctor does, but your doctor knows more about bodies in general than you do, so you have to work together.
That's well said, as far as it goes, but I wouldn't leave out the point about different incentives and levels of investment in the outcome for the two parties. (The pithy, cynical versions: "First, let them get sick" (Seth Roberts) and the life/dinner principle (Dawkins and Krebs).)
