First off, mental health professionals have not used the term sane or insane for at least 70 years, the term is now only used in legal settings and has no meaning or relevance to psychology or psychiatry.
It is my opinion that behavior and thoughts allow mental health professionals to categorize someone as sane or insane. i dont believe such a distinction is valid, as mental health is complex, and not dichotomous like they would have you believe.
Secondly, I agree. Mental health is definitly not dichotomous and any psychiatrist/doctor/phsycologist who believes it is should lose their degree. But i dont believe most believe it is. For instance Even the DSM recognizies the complexities of diagnosing mental disorders. A recent change to the next edition of the DSM(I think this is true correct me if im wrong) will be the removal of aspergers and the inclusion of an autistic spectrum(which will include aspergers), hence the DSM is recognizing the huge variety and complexity of autism and the fact that it is not a single disease much alike to schizophrenia, which is also considered a spectrum of diseases rather than just one.
I agree with both sides of this argument, but there are ways to look inside the brain in fact there are many ways: MRI, fMRIs, ECG, CT or CAT and many others ways to look inside the brain. And in a lot of mental illnesses they have shown differences between the brains of people with a mental illness and people without, for instance the frontal lobes are indicated in ADD/ADHD and a structure(i forget which one) within part of the frontal lobes is abnormal within an ADD/ADHD mind, they have done brain imaging with major depression and discovered subtle differences/abnormalities compared to a "normal brain" too.
Also Psychology does deal with the brain in fact there are many fields focused on the brain within psychology, probably the most relevant would be
NEUROPSYCHOLOGY! its a huge developing field where they study the brain in great detail and how human behavior relates to brain structures. I am currently enrolled in an undergraduate course on Neuropsychology.
If a doctor could look at a person's brain, point out an area where there is a physical, tangible difference and say "this is why this person has ADD/depression/anxiety" I would agree and say it is an illness. Otherwise, I look at it as strange behaviors due to something possibly in the person's upbringing.
^They can look at a person's brain and as i explained before there are tangable differences between ADD/(major)depression/anxiety(Generalized, PTSD, chronic..) and a "normal" brain.
To say that psychology doesn't look at or deal with the brain is ridiculous... it is just that in most cases a patient or a client cannot afford a MRI or other brain imaging technique so they base the diagnosis off of reported symptoms and observed behavior.
That being said I do believe that many many drugs like SSRI (and other anti-depressants), benzos, and amphetamines are over prescribed and are not a cure for an illness, in many cases they hinder progress. I also agree with you that people who may show eccentric or antisocial(not the personality disorder, but antisocial as in shy, introverted, nontalkitive.) tendencies are given drugs to "normalize" them when they should not be. Whats going on today with children and ADD/ADHD diagnosis in this country is appaling(and reckless, disgusting, inhumane even) and seems like an attempt for parents/society to control there childs behavior.
Finally one of the basic concepts behind psychology is the nature vs. nurture philosophy: For example when looking at twin studies(identical, gentically clones) with schizophrenia spectrum disorders, if one twin has schizophrenia the chances the other twin will have it is very high, but if the twins were raised seperatly by different parents and again if one twin has schizophrenia, the chances of the other twin having it are lower, than if they were raised together. But the chances of the twin, whose other twin has schizo.., and was raised seperatly still has a much greater chance of having becoming schizophrenic than the average person. Hence there is evidence that schizophrenia is partly genetic (nature) but also life events, upbringing and personal history/story (nurture) plays a large role in whether someone will become schizophrenic or not. One of the most important concepts within psychology is that no illness/disease/disorder is all nature or all nurture its a mixture of the two.
here is a link to what i was talking about, has pictures that show obvious differences in the brains of identical twins one with schozophrenia and one without:
http://www.schizophrenia.com/research/hereditygen.htm