There is a quite good article discussing the differences between physical diagnoses and psychiatric diagnoses on this website. A one sentence summary: you have physical diseases, you do psychiatric illnesses. Most people think they have, say a depressive illness. Not so, they did it in front of a psychiatrist or other psychiatric diagnostician. Then he/she put a label on what the patient did, said, looked like during an interview, say anxiety reaction.

In contrast we do have physical illnesses. While some observation is involved the diagnostic process this is backed by lab tests, Xrays etc. which are far more stable than what any of us do, say, act out from moment to moment or hour to hour. I remember studies being done back in the 1950's demonstrating that 2 psychiatric diagnosticians frequently come up with very different diagnoses when seeing the same patient in the same half day. That happens far more seldom with physical diagnoses.

I believe the 2 paragraphs above are true, factual. If I'm wrong I would appreicate being corrected, with explanations, reasons why I'm wrong. Even better would be a reference or two to studies demonstrating I'm wrong. I'll be here a while.