Functionalism takes various forms, bu tin general it is the view that mental states are defined by their causes and effects. Thus, what makes an inner state mental is not an intrinsic property of the state,but rather its relations to sensory stimulation (input), to other inner states,and to behavior (output). For example, what makes an inner state a pain is its being a type of state that is typically caused by such things as pinpricks,and typically causes certain behavior (e. g. , saying ouch ). To be in mental state type m is to be in a physical state n that typically occupies causal role r. All versions of functionalism face problems about the qualitative nature of mental states. Most of use want to make the distinction between ourselves as subjects (perceivers) and the rest of the world---the objects of our experiences or perceptions (what we perceive)----the physical world around us.