Functionalism originally appeared to be just a way of meeting objections to the psycho-physical identity thesis. Then it was suggested that functional relationships actually constituted the nature of mental states. As an objection to functionalism the alleged possibility can be treated in either of two ways, as for the case of absent qualia. One is to follow Dennett and deny that it is a genuine possibility at all. The other is to adopt 'deep' functionalism and maintain that a case of transposed qualia could be underlain by internal functional differences, hence is no problem for functionalism in general. The idea of absent qualia in general is compatible with functionalism, and is to be sharply contrasted with the idea of zombies. According to functionalism, whether or not an individual is in a given mental state depends on causal or other relations between that mental state and inputs, outputs and other mental states.