Someone who is hallucinating will see things, whole scenes and events which are not actually there, and experience them as real.How can this be? Because you see what you expect, unless it is corrected by your perceptions. Which is where things can go wrong.
Disease can be seen as the antechamber of death.
Are there reliable tests for predicting disease? The short answer is: not for Alzheimer’s or other common diseases. Tests do exist for some specific diseases, most with known familial predispositions, such as Huntington’s disease, cystic fibrosis, Duchenne’s dystrophy and many more.
What causes your depression: serotonin or what you experience? The answer: the question is invalid! Both play a role, and are interconnected. You cannot say that one of these is the cause.
A lot of people think antidepressants are worthless. Is psychotherapy better than an antidepressant? There is no evidence for this. It seems to me that antidepressants suffer a disadvantage in our perception as compared to psychotherapy, in that their mechanism of action is quite invisible and not instinctively comprehensible.