So, there is no such thing as a mental illness! Well, is there such a thing as physical illness? Paralysis is a form of illness. You were paralyzed when a tree fell on you while running and broke your spine. So we have now established that there is such a thing as physical illness. You have one! So we have also established that the physical body can break down and get sick.
Now let us ask whether the brain is part of the physical body. Well is the brain a part of the physical body? Yes, it is. The physical body can get sick, the brain is part of the physical body, therefore the brain can get sick! What happens when the brain gets sick? Well, the brain can get cancer, as in tumors. It can produce hormonal imbalances, as in diseases of the pituitary gland. It can result in illnesses such as Parkinson’t disease, a physical illness of the brain tissue called substantia nigra which is responsible for producing dopamine, a neurotransmitter. Neurotransmitters are molecules through which neurons communicate with each other and through which neurons communicate with other cells of the body, like muscle cells. Neurotransmitters are also involved in the production of feelings, emotions, thoughts. When neurotransmitters (either levels or types) go awry, we have a chemical imbalance that then results in symptoms of mental illness. Ostensibly, too much dopamine can cause aggression and hallucinations, too little serotonin can cause depression and muscle aches. There is GABA, nitrous oxide, epinephrine/adrenaline and norepinephrine/noradrenaline. And an imbalance in any of these neurotransmitters (and in others not yet known) can cause not only physical symptoms, but emotional, mental, and thinking disorders/illnesses.
So there it is, the human body can get illnesses, the human brain is part of the human body, therefore it too can get sick. Some of the illnesses that the human brain gets are mental illnesses, diseases of the emotions, thoughts, and feeling, as the brain is the seat of these things.
Perhaps, if you had appointed someone who believed in science and evolution as the chair of your state board of education, instead of the laughable Barbara Cargill, you might well have learned this.