Is there such a thing as mental illness?

Hungarian-American psychiatrist Thomas Szasz argues there is no such thing as mental illness. Is he and his ilk right?

Pro

 * Mental illnesses are recognized as medical illnesses by a range of reputable sources.
 * That does not provide a substantive proof or analysis, merely an argument from authority.
 * Some behaviors are obviously deranged enough as to count as diseased/ill, whether the cause is a brain disease or something else.
 * That does not substantiate the claim that there exist distinct well-defined disease entities in the field of human behavior, analogous to medical diseases (being distinct entities) such as cancer and its forms, virus-caused disease or bacteria-caused disease.
 * The existence of distinct entities is not the motion, merely that some people are mentally ill.
 * Whatever kind of entities the so-called mental illnesses are, the fact that the symptoms respond to medication suggests they are medical illnesses/diseases.
 * Anyone emotionally disturbed by adverse events in life is expected to have the behavior modified when given major tranquilizers (also known as antipsychotics), but that does not prove there is an illness.
 * Emotional and cognitive states are modified in response to alcohol intake, but that does not mean that these pre-alcohol-intake states are an illness and alcohol is a medicine.

Con

 * There is no such thing as mental illness per Thomas Szasz..
 * That is not a specific argument.
 * The arguments are as if included by reference.