Doctors and Mechanics
Here's an interesting fact: In one small way, a doctor and an auto mechanic are the same.
That similarity lies in the basis of the art and science of diagnosis:
You have to know what it's supposed to do in order to diagnose what it's doing wrong.
As an example, in diagnosing the front suspension/steering on my '07 Cobalt, I was at a real disadvantage: I didn't know what it drove like new, so how exactly was I supposed to know what idiosyncrasies were in the design and which ones were driving problems?
Obviously I couldn't, so I ended up having everything in the front end replaced — well everything sans steering rack, it was fine. Luckily, everything in the front end was bad. So, win there...I guess.
Another example: if you have an old Camaro, Mustang, or Dart and you think there is an idle problem, do you know? Case in point, base model 289s in Mustangs idle smooth, but Shelbys idle rough — and the lifters rattle. If you didn't know that and thought your Shelby ran like shit, you would be wrong. It's supposed to be that obnoxious.
That really is the crux of all diagnosis, regardless of whether it's a car, a person, or a blender: in order to properly diagnose the problem, you have to know what right is before you can know what wrong is.