A good starting point would be to define Mental Illness. In the DSM you see everything from identifiable diseases with etiologies, to much more broadly defined syndromes with multiple supposed potential causes that go from genetic to environmental. So what counts; is incessant fidgeting a mental illness or is it someone who was just never disciplined as a kid? Is psychopathy an illness or is it just someone who was born way to one side of the mental spectrum, with the same mix of psychological traits we all have, merely in massively different balance and proportion?
And the follow up question is does it really matter in the end? It’s the old legal debate; what’s the practical difference between an irresistable impulse and an impulse that wasn’t resisted?