Mental Illness as a Category of Its Own

Despite attempts to redefine mental illness, it exists as its own category.

Posted Nov 27, 2019

U.S. Senator Kamala Harris, currently a candidate for the Democratic presidential nomination, recently introduced her plan to address the nation's ongoing mental health crisis. As part of her plan, Harris proposes reclassifying serious mental disorders like schizophrenia and bipolar disorder as neurological diseases rather than mental illnesses.

Mark L. Ruffalo, D.Psa., L.C.S.W.
Source: Mark L. Ruffalo, D.Psa., L.C.S.W.

Harris is not alone in this regard. For years, advocates for the seriously mentally ill have called for a reclassification of serious psychiatric disorders as "brain disorders." Thomas Insel, former chief of the National Institutes of Mental Health, famously proclaimed that mental illness is actually brain disease. The Nobel laureate Eric Kandel has called for a merging of the fields of psychiatry and neurology.

I contend that such proposals are based on a classic category error and ignore the historical, philosophical, and scientific reasons for the existence of a separate category of disease known as "mental illness."

While it is logically true that what we call "the mind" is a product of the physical brain, it does not necessarily follow that diseases of the mind (mental disorders) are best understood as diseases of the brain. Certainly, any abnormal mental state has some underlying neurological correlate, but mental illnesses are so-called because they affect mentation, emotion, and behavior. In the sense that mental illnesses often affect behavioral conduct, they are sometimes referred to as "behavioral disorders," and psychiatrists as experts in "behavioral medicine."

In contrast, neurological diseases do not chiefly affect human behavior and cognition, though sometimes they do. Neurological diseases like Parkinson's disease and epilepsy can be identified via biological means and are thus best understood as structural disorders. Their impairment of thought and behavior is secondary to their deleterious effects on the nervous system. Mental disorders, however, have historically been classified as functional disorders, manifesting chiefly as alterations of human emotion, perception, and conduct. The mental illness conversion disorder, now also referred to as functional neurological symptom disorder, demonstrates this distinction between structural and functional disease.

Furthermore, mental illnesses have been clearly demonstrated to bear some relation to a person's environment. An abundance of research over the past 100 years has shown that environmental and psychological factors play a role in the development and maintenance of mental illness. Certain types of life experiences can influence the onset and course of mental disease. In contrast, there is no evidence to suggest that environmental-psychological factors have any significant influence on the development of neurological disease. Psychotherapy can treat mental illness, but it makes no sense to speak of psychotherapy as a treatment for brain cancer or stroke.

It is clear, then, that the boundary between psychiatry and neurology is not an artificial one. This does not mean that mental illnesses are unrelated to the brain or that brain science plays no role in the understanding and treatment of mental illness. To the contrary, many major advances in psychiatry are attributable to neuroscience. Nor does this mean that mental disorders are not diseases. The concept of disease in medicine has always been associated with suffering and incapacity, and the classification of an entity as a disease does not require a known, or even putative, biological cause (see Pies, 1979).

The reality is that "mental illness" and "brain disease" are complementary, not contradictory, categories. Many neurological diseases manifest with mental symptoms; and some mental disorders, such as Alzheimer's disease, have known neuropathology, but remain classified as mental illnesses (see Ruffalo & Pies, 2018). Yet, two categories of illness exist: mental and neurological. Their distinction is not merely arbitrary.

As psychiatrist Ronald Pies, M.D. has astutely noted, the discovery of the biological basis for all mental disorders would not eliminate the category "mental illness," nor would it negate the use of mentalistic language to describe these disorders. Regardless of future advances in neuroscience, there will always exist a category of disease known as "mental illness" that is separate from, but not mutually exclusive of, neurological disease.

Psychiatrists and psychotherapists will always be experts on diseases that reside at the interface of an individual's biology and their outside world, affecting the range of human mental functioning. This is what makes psychiatry unique, and it is precisely why there exists a separate category of disease called mental illness.

For a more thorough discussion of this topic, please see the paper Mental Disorders are Not Brain Disorders by Natalie F. Banner, Ph.D., published in the Journal of Evaluation in Clinical Practice in 2013.


Pies, R. W. (1979). On myths and countermyths: More on Szaszian fallacies. Archives of General Psychiatry, 36(2), 139-144.

Ruffalo, M. R., & Pies, R. W. (2018). The reality of mental illness: Responding to the criticisms of antipsychiatry. Psychology Today. Retrieved from