I've seen the term "mental illness" used a lot over the past few days. This reminds me of our problems with language, conceptualization and classification of psychological "disorders." There are serious problems with our systems of classifying and labeling, but I'm only going to say a bit about the popular term "mental illness" and the word "disorder," the latter being preferred by many clinicians.
One problem with the term "mental illness" is that we might incorrectly infer from it that discrete, clinical entities exist, each with a specific and separate underlying physiology. The term would be better thought of as a metaphor rather than a concrete state of affairs.
It might also be assumed that there is something wholly objective about the word illness applied to certain collections of symptoms and conditions of mental life. That would be an unwarranted assumption. Disorder is probably preferred by some clinicians because it sounds more agnostic on the subject of physiology, but the determination of what constitutes a mental disorder is no more objective than the designation mental illness, notwithstanding the DSM checklists.
Aside from the suggestion of physiological meaning, the term mental illness has social meanings and implications. It colors our view of responsibility and it may serve to lessen social stigma*, but it ignores the power of the unique personal narratives behind our subjective view of the world and ourselves. And it's within the field of the subjective--our point of view--that change can occur through interactions between people. Consider this question: how many illnesses can be treated** by talking and sharing point of view?
I'm not saying that we should accept a strict mind-body dualism, but we should think about the language we use, and recognize that our constructs have limitations. This is true whether we're talking about developmental conditions, personality disorders, acute psychological conditions or psychoses.
* The word illness also means health care providers will pay for treatment.
** Even the word treated is loaded with meanings that can be misleading.