> When the client tells you what the cause is, he's wrong.

using a software analogy, I've seen plenty of bug reports where the reporter did some research on his own that turned out to be determinant in the issue resolution. You have training, inside knowledge of the system that's failing, but you see plenty of issues everyday, cannot dedicate to each one the time they would deserve. The reporter instead is motivated to reach a solution and has plenty of time to research it properly. Discarding a potentially fundamental input just because it comes from someone supposedly untrained would be a shame.

>Tell the doctor what you've observed, let him conclude the diagnosis.

Lots of anecdotal stories of people doing just that and having doctor after doctor failing them.

Indeed. My hypothyroidism was ultimately diagnosed by my mother's work colleague who recognized the (frankly quite textbook) symptoms because her dog had it.

Doctors are like everyone else: 90% of them are kinda shit at their job.

Feynman's wife.