Mental illness does indeed exist. There are many who claim it does not, but then the historical and modern-day study of the brain and all the minute details of it have been for nothing. Mental illness is a deviation from what is accepted as "normal" behavior, and can have many causes.
Scientists, the researchers in the field of psychiatry and psychology, have devoted their entire careers to studying the brain. They begin with lower level courses in physiological psychology, which attempt to explain the connection between the medical model and the syndromes in mental illness that stem from that connection. The brain and spinal cord hold the answers to many of the mental disorders that plague patients and clients with mental conditions. That connection to the rest of the body is what explains how mental illness does exist and why.
These scientists explore the brains of animals as a beginning step to test their various hypotheses about workings of the brain - how it reacts to certain stimulae, to medications, to removal of tumors, to creation of lesions - and to behavior, recovery, and long-term effects of these experiments on the animals. Contrary to popular belief, animals used for these studies are humanely treated and kept from suffering. Labs must adhere to strict standards for humane treatment of research animals.
Moving forward to actual use of medical procedures in humans, at a certain point patients are permitted to opt for treatment with an experimental drug. This is where treatment is shown to be efficacious or not. Scientific studies help researchers document their findings and follow-up with longitudinal studies to see how clients are doing after specific periods of time.
If we say there is no such thing as mental illness, we are ignoring the hard work of our scientists and researchers who have dedicated their entire lives to finding not just cures, but reasons behind the bizarre and troubling symptoms of those with mental illness. Those who have been diagnosed as mentally ill would not have any hope for the future if this were the case. As new medicines and diagnostic procedures are discovered and put into use, sufferers have more hope for a return to productive lives.
Sadly, there are no cures for many forms of mental illlness yet. The best we can do is keep symptoms under control so as to keep mentally ill patients and their loved ones (and society) safe and comfortable. Sufferers of Alzheimer's Disease, schizophrenia, bi-polar disorder, and many other forms of mental illness can only hope to have their symptoms contained, life-expectancy prolonged, and with proper treatment, enjoy a more normal way of life within society. With new and advanced medications that control the interactions between the brain and the rest of the body, monitoring of hormone levels that regulate the brain's activity, and more advanced diagnostic tools, we are glad for the advances that have been made to keep symptoms at manageable levels.
There is no denying mental illness. Because "brain learning" tells us that we use our brain for everything we do, and everything we do is controlled by the brain, it stands to reason that the rest of the body takes its cues from the messages coming directly from the brain. This knowledge is all the evidence we need to understand that in the same way physical illness exists, so does mental illness. The existence of mental illness as a reality as valid as physical illness requires one to "connect the dots."