Behaviour therapy is a form of psychotherapy use to treat depression, anxiety disorders, phobias, and other forms of psychopathology. Its philosophical roots can be found in the school of behaviorism, which states that psychological matters can be studied scientifically by observing overt behavior, without discussing internal mental states. Without holding inter states as causal, Skinner's radical behaviorism accepted internal states as part of a causal chain of behavior but continued to hold that the only way to improve the internal state was through environmental manipulation.