According to Lawrence Stevens, J.D. “All diagnosis and treatment in psychiatry,
especially biological psychiatry, presupposes the existence of something called
mental illness, also known as mental disease or mental disorder.” I’ve brought
this up before, what exactly is meant by mental disease, illness, or disorder?
In a semantic sense disease means simply disease, the opposite of ease. But by
disease we don’t mean anything that causes a lack of ease, since this definition
would mean losing one’s job or a war or economic recession or an argument with
one’s spouse qualifies as “disease”. But clearly this is not what is meant by
“disease.”

disease, illness, or disorder means that the body
is not functioning in its normal, expected, or optimum way.
Sources of illness include trauma (injury), infections (bacteria
and viruses), growth of abnormal tissues (tumours), poisoning,
abnormal body chemistry usually from birth (e.g. cystic fibrosis),
and attacks by the immune system on friendly tissue.

There are diseases of the brain which produce gross changes in
function e.g. epileptic fits, or gross changes in tissue e.g.
tumours and scarring from injuries.

The diseases referred to as “mental illnesses” are really illnesses
of behaviour: a person find, to their distress, that quite ordinary
pressures make them fall into weeping sadness, for example. They are
diseases of brain function. Indeed, we can look at an MRI scan of
a living brain, and see that it is operating in rather different
ways between a schizophrenic and a normal subject.

It is simply no longer true that there is “no detectable physical
sign associated with diseases of behaviour”.