Intentional states are multiply realizable, and functionalism was motivated by this fact. The supervenient nature of intentional states constitutes a real block to reductive materialism for intentionality. However intentional states can be individuated operationally. The intuition that the psychological description “He likes chocolate” involves a reference to the subject’s qualitative experience of tasting chocolate is wrong (or, we can mention the qualitative experience but we cannot actually convey it). In the case of the Martians we might not know if they even “taste” things at all; nonetheless we might come to know that they like chocolate.
A consequence of the necessarily operational basis of intentional descriptions is that, to use an example made famous by Daniel Dennett (although I don’t know that Dennett would agree with my line here), a lowly thermostat is a kind of intentional system: we can determine when it thinks that the room is too cold, just right or too hot. The intuition that this can’t be, that a thermostat is clearly not a mind, is a consequence of internalizing the traditional homogeneous concept of “mind” (because the thermostat has no Nagelian experience), aggravated by the prevailing dogma that thinking necessarily involves representations. When we disambiguate “mind” and see that intentionality is something altogether different than consciousness there is no denying that the thermostat is, in fact, an intentional being; nor does that fact in any way compromise our philosophical use of the term “intentionality.”
The qualities of experience, on the other hand, are not supervenient. It is plainly true that humans, dolphins, probable intelligent extraterrestrials and possible intelligent artifacts, among an indefinitely large set of other beings, can all believe that the chocolate is in the box, desire the chocolate and so forth. But there is no reason whatever to think that chocolate tastes like that (the way it tastes to me, say) for all of the members of the set: there isn’t even any reason to think that all of the members of the set of chocolate-desirers taste anything at all.
Ironically what this amounts to is that intentional properties are more ontologically mysterious, not less, than phenomenal properties. Consciousness has been called the “hard problem,” but in fact the right metaphysical account of consciousness is, relative to that of intentionality, positively straightforward.