He stated that a balance between the mind and body is required for good health and that an imbalance between the two can cause sickness. Al-Balkhi also introduced the concept of reciprocal inhibition (al-ilaj bi al-did), which was re-introduced over a thousand years later by Joseph Wolpe in 1969. Psychophysiology and psychosomatic medicine The Muslim physician Abu Zayd al-Balkhi was a pioneer of psychotherapy, psychophysiology and psychosomatic medicine. He recognized that the body and the soul can be healthy or sick, or "balanced or imbalanced", and that mental illness can have both psychological and/or physiological causes. He wrote that imbalance of the body can result in fever, headaches and other physical illnesses, while imbalance of the soul can result in anger, anxiety, sadness and other mental symptoms. He recognized two types of depression: one caused by known reasons such as loss or failure, which can be treated psychologically through both external methods (such as persuasive talking, preaching and advising) and internal methods (such as the "development of inner thoughts and cognitions which help the person get rid of his depressive condition"); and the other caused by unknown reasons such as a "sudden affliction of sorrow and distress, which persists all the time, preventing the afflicted person from any physical activity or from showing any happiness or enjoying any of the pleasures" which may be caused by physiological reasons (such as impurity of the blood) and can be treated through physical medicine. He also wrote comparisons between physical disorders with mental disorders, and showed how psychosomatic disorders can be caused by certain interactions between them. Phobias Abu Zayd al-Balkhi initially wrote about phobia in the 9th century, calling it Fazaa'. As of today, it is still defined in a similar manner as to what was described by al-Balkhi. Al-Balkhi details the disorder as an extreme type of fear that results in the anxiety levels of a person spiking due to increasing the depth of blood in the body. This causes the person to become shaky and disoriented, preventing them from making decisions in a timely manner. Rather than taking medicine or proceeding with blood letting, which were common methods to help an individual, he suggested gradual exposure to the object or situation that caused the fear. This technique was known as reyadat al-nafs. Sexual attributes While the topic of sex is more widely discussed today, al-Balkhi explored the subject in detail,