there are no mental illnesses that can be diagnosed on any basis other than an assessment of behaviour. There is no bug, no gene, no chemical deficiency, no physical property at all that unequivocally marks out any psychological illness. — unenlightened
True, and I don't think that genetics is anywhere close to parsing out all the genetic factors that control the behavior of the brain for learning 2 + 2 = 4, let alone somebody who thinks they are Jesus.
I knew a guy who claimed to be Jesus. (This was not in a psychiatric setting,) He was educated, charming, intelligent, funny, apparently grounded in reality, and so forth. There are three possibilities. A) he was pulling my leg. 2) Perhaps he was Jesus. Why not? 3) Perhaps he was a perfectly functional lunatic.
Why should there be some other method of diagnosing a mental illness other than observing behavior? If this guy thought he was Jesus, what more would you want to see in a blood test or MRI? A guy who thinks he is Jesus but is also witty, urbane, intelligent, educated, and an effective employee raises questions that behavior answers, it seems to me: He may be deluded, but he is otherwise behaving normally. So who cares if he thinks he is Jesus? It's more interesting than being a run of the mill bat shit white supremacist, don't you think?
If a patient is delivered to the emergency room of a hospital after being found on the street naked, screaming incoherently, displaying intense agitation, displaying inability to interact with the staff, what more than behavior observation do you need? Severe mania and psychosis produce this kind of behavior. The pt's blood can be checked for hallucinogens. No drugs? It's probably mania.
How long does the mania last? Does the pt. respond to major/minor tranquilizers or anti-seizure drugs? How long does the pt. stay awake in this condition? Doesn't respond to drugs? periodically and suddenly falls asleep wherever he happens to be? Better do a neurological work up.
Medicine -- including psychiatry -- has a very strong element of "practice". After seeing 500 pts. in mania, a pattern probably becomes pretty clear. How many moles does a dermatologist have to look at before than can say at a glance, "malignant" or "nothing to worry about". After a while they know the drill. Untextured red spot on skin? Probably cancer. Textured red spot on skin? Probably benign. Patient is counting the ceiling tiles in the waiting room. Probably OCD. Your friend has to check to make sure the stove is turned off, the refrigerator is closed, and the door is locked 5 times before than can leave. Definitely OCD. No need to see a psychiatrist.
But those are the easy kinds of cases.