But while we don't know whether or not cholesterol can be loweed by proper diet, we do know that cholesterol exists. We don't know if mental illness exists. We are reasonably sure that OCD and bipolar exist, but only reasonably sure, not certain. So you would be wrong to respond to a question about the causes of mentalillness as you would be wrong in responding to a question about the differences between vegetable and mineral. Those terms don't name anything but merely refer, vaguely, to a collection of quite different things. The same is probably true of "mental illness." Only probably. They may discover some day that the different mental illnesses under that term are only variations on a single 'illness," which will then be given a proper name. Or they may discover that "Depression" doesn't exist, only three or four or more quite differenct "illnesses" which have been improperly huddled together under the term "depression."
Carrol