One of the ways in which behaviourists treat phobias is by using systematic desensitisation. This process is based on classical conditioning (counterconditioning). The treatment involves replacing an unwanted response to a stimulus (e.g. replacing fear with relaxation) as fear and relaxation cannot coexist. They formulate a hierarchy of anxiety and gradually expose the individual to the different stages, starting at the lowest level of anxiety. Once comfortable, they proceed to the next level until the phobia is extinct.