> If science had found biological origins or symptoms for mental illness in the brain, don't you think these would be used for diagnostic purposes?

Science has found biological symptoms for Alzheimer's and rabies. Unfortunately, these can only be firmly identified post-mortem and are thus not useful for diagnostic purposes. Just two examples if diseases with biological markers that are not useful.

> For instance, did you know that he was the first psychiatrist to claim publicly that homosexuality is not a disease? (In 'Sexual Inversion: The Multiple Roots of Homosexuality', ed. by Judd Marmor, Basic Books, 1965)

A broken clock is right twice a day. When you claim mental illness doesn't exist in toto, then of course you will be "right" about bullshit mental illnesses like homosexuality.