Behaviourists are therefore right in excluding consciousness, purpose, and mind from psychology. These are inaccessible to science: first, because they can never be precisely seized; and second, because they introduce an indefinite number of objects which preclude a limited and stable environment. Mind can reflect upon all objects of experience, and these in turn can become objectives of action. So soon as this happens the exact control which experimental science demands is prevented and the progress of psychology is blocked. It thus seems impossible for psychology to deal with the most interesting aspect of human behaviour, those where action is guided and dominated by thought and is not blindly reflexive. To avoid this limitation of psychology certain behaviourists, in the name of what they conceive to be mechanism, have denied the efficacy of purpose and mind altogether. According to the argument, consciousness, purpose, and mind are waste products, attendant upon natural processes, like sparks from a locomotive. Or they are like the decorations on illuminated manuscripts, illustrating the text, and quite superfluous. Otherwise they would be forces, inexplicably interfering with natural processes. These arguments fail by misconstruing the notion of purpose, which is not a force but an end. Purpose does not direct action but _is_ the direction which action takes. It is the rational explication of the objects of our desires and wishes. The end has no initial power, but, once thought has discerned it as the true object of our desires and they have attached themselves to it, as a lover to his mistress, so that satisfaction cannot be found elsewhere, then the end acquires a deputed power which it may never surrender. Since purpose defines the objects of desire which thereby become objectives of action, it is a determining factor of human behaviour, and the so-called mechanistic arguments against it are invalid. Yet, since science deals with material and efficient causes only, psychologists are compelled to disregard formal and final causes, purposes, and ends. II