- the symptoms of schizophrenia, bipolar, autism, and, for all I know, everything up to a fondness for romance novels are the behavioral manifestations of biological things going on in brains, and
- much of that is controlled by genes, which alleles are present and which are expressed, and
- the genes and neuroscience do not map straight across to symptoms,
therefore, it makes sense to describe mental illnesses in terms of the biological substrate rather than the behavior.
This all makes sense to me. When you deal with a person with a mental disorder face to face, you see the symptoms, but if you're in the lab trying to figure out how to fix or ameliorate the problem, you're better off thinking in terms of problems with myelination, expression of certain genes, or uptake rate of some neurotransmitter. The rest of us can still think in terms of schizophrenia, bipolar, etc, but we may need to get used to a different set of scientific categories.