A doctor was running a final test to determine whether the treatment for his psychiatric patients works or not

So the doctor brought the 3 patients to an empty pool and asks all of them to jump into the pool.

The first one without hesitate jumped and landed face first thus considered fail.

The second one took a step back, and looked at the doctor, and he said 'doc I cant swim, I might drown'. The doctor asked him to jump, and he jumped. Thus failed too.

The last one refused to jump and the doctor actually thought he had passed the test. The doctor forced him to jump and yet he still refused. Feeling that he might just have cured the last patient's insanity, he asked 'why dont you want to jump?'

The patient dipped his finger into the non-existent water and said to the doctor seriously 'The water is fucking cold!'