Multiple realizability One of the most influential and common objections to the type identity theory is the argument from
multiple realizability. The multiple realizability thesis asserts that mental states can be realized in multiple kinds of systems, not just brains. Since the identity theory identifies mental events with certain brain states, it does not allow for mental states to be realized in organisms or computational systems that do not have a brain. This is in effect an argument that the identity theory is too narrow because it does not allow for organisms without brains to have mental states. However,
token identity (where only particular tokens of mental states are identical with particular tokens of physical events) and
functionalism both account for multiple realizability. The response of type identity theorists, such as Smart, to this objection is that, while it may be true that mental events are multiply realizable, this does not demonstrate the falsity of type identity. As Smart states: :"The functionalist second order [causal] state is a state of having some first order state or other which
causes or is
caused by the behavior to which the functionalist alludes. In this way we have a second order type theory". The fundamental point is that it is extremely difficult to determine where, on the continuum of first order processes, type identity ends and merely token identities begin. Take Quine's example of English country gardens. In such gardens, the tops of hedges are cut into various shapes, for example the shape of an elf. We can make generalizations over the type
elf-shaped hedge only if we abstract away from the concrete details of the individual twigs and branches of each hedge. So, whether we say that two things are of the same type or are tokens of the same type because of subtle differences is just a matter of descriptive abstraction. The type-token distinction is not all or nothing. essentially rejects functionalism because, he believes, it is indeed a second-order type identity theory. Putnam uses multiple realizability against functionalism itself, suggesting that mental events (or kinds, in Putnam's terminology) may be diversely implemented by diverse functional/computational kinds; there may be only a token identification between particular mental kinds and particular functional kinds. Putnam, and many others who have followed him, now tend to identify themselves as generically
non-reductive physicalists. Putnam's invocation of multiple realizability does not, of course, directly answer the problem raised by Smart with respect to useful generalizations over types and the flexible nature of the type-token distinction in relation to causal taxonomies in science.
Qualia Another frequent objection is that type identity theories fail to account for phenomenal mental states (or
qualia), such as having a pain, feeling sad, experiencing nausea. (Qualia are merely the subjective qualities of
conscious experience. An example is the way the pain of jarring one's elbow
feels to the individual.) Arguments can be found in
Saul Kripke and
David Chalmers, for example, according to which the identity theorist cannot identify phenomenal mental states with brain states (or any other physical state for that matter) because one has a sort of direct awareness of the nature of such qualitative mental states, and their nature is qualitative in a way that brain states are not. A famous formulation of the qualia objection comes from
Frank Jackson in the form of the
Mary's room thought experiment. Let us suppose, Jackson suggests, that a particularly brilliant super-scientist named Mary has been locked away in a completely black-and-white room her entire life. Over the years in her colour-deprived world she has studied (via black-and-white books and television) the sciences of neurophysiology, vision and electromagnetics to their fullest extent; eventually Mary learns all the physical facts there are to know about experiencing colour. When Mary is released from her room and experiences colour for the first time, does she learn something new? If we answer "yes" (as Jackson suggests we do) to this question, then we have supposedly denied the truth of type physicalism, for if Mary has exhausted all the physical facts about experiencing colour prior to her release, then her subsequently acquiring some new piece of information about colour upon experiencing its
quale reveals that there must be something about the experience of colour which is not captured by the physicalist picture. The type identity theorist, such as Smart, attempts to explain away such phenomena by insisting that the experiential properties of mental events are
topic-neutral. The concept of topic-neutral terms and expressions goes back to
Gilbert Ryle, who identified such topic-neutral terms as "if", "or", "not", "because" and "and." If one were to hear these terms alone in the course of a conversation, it would be impossible to tell whether the topic under discussion concerned geology, physics, history, gardening, or selling pizza. For the identity theorist, sense-data and qualia are not real things in the brain (or the physical world in general) but are more like "the average electrician." The average electrician can be further analyzed and explained in terms of real electricians but is not itself a real electrician.
Other Type physicalism has also been criticized from an
illusionist perspective.
Keith Frankish writes that it is "an unstable position, continually on the verge of collapsing into
illusionism. The central problem, of course, is that
phenomenal properties seem too weird to yield to physical explanation. They resist functional analysis and float free of whatever physical mechanisms are posited to explain them." He proposes instead that phenomenality is an illusion, arguing that it is therefore the illusion rather than phenomenal consciousness itself that requires explanation. ==See also==