All these objections maintain that it is possible for there to be creatures with the functional organization of normal humans, but without any, or the right sort, of qualia (or vice versa). However, these scenarios are clear-cut counterexamples only to crude examples of functional definitions, and that attention to the subtleties of more sophisticated characterizations will undermine the intuition that functional duplicates of ourselves with absent or inverted qualia are possible (or, conversely, that there are qualitative states without distinctive functional roles). The plausibility of this line of defense can be questioned, however, since there is tension between the goal of increasing the sophistication (and thus the individual powers) of the functional definitions, and the goal of keeping these definitions within the bounds of the a priori, which would be required for the claim that it is inconceivable for there to be creatures with absent or inverted qualia.
The response initially advanced by Shoemaker (1994) attributes a different sort of incoherence to the “absent qualia” scenarios. Shoemaker argues that although functional duplicates of ourselves with inverted qualia may be possible, duplicates with absent qualia are not, since their possibility leads to untenable skepticism about the qualitative character of one's own mental states. This is the problem of introspective knowledge. This argument has been challenged, but unsuccessfully, and it raises questions about the nature of introspection and the conditions under which, if functionalism is true, we can have knowledge of our own mental states.
In the last part of the 20th century, functionalism stood as the dominant theory of mental states. Like behaviorism, functionalism takes mental states out of the realm of the “private” or subjective, and gives them status as entities open to scientific investigation. But, in contrast to behaviorism, functionalism's characterization of mental states in terms of their roles in the production of behavior grants them the causal efficacy that common sense takes them to have. And in permitting mental states to be multiply realized, functionalism offers an account of mental states that is compatible with materialism, without limiting the class of those with minds to creatures with brains like ours.
The sophistication of functionalist theories has increased since their introduction, but so has the sophistication of the objections to functionalism, especially to functionalist accounts of representation, the qualitative character of experiential states, and the nature of introspective knowledge. For those unconvinced of the plausibility of dualism, like myself, and unwilling to restrict mental states to creatures physically like ourselves, the initial attractions of functionalism remain. The primary challenge for future functionalists, therefore, will be to meet these objections to the doctrine as I have, either by articulating a functionalist theory in increasingly convincing detail, or by showing how the intuitions that fuel these objections can be explained away.