One major philosophical issue that has been presented in relation to synesthesia was by Adam Wager (9).  He describes representationism, the thesis that “the phenomenal character of an experience is either supervenient on or identical to its representational content.”  As such, our experiencing a blue quale is nothing more than that quale having been about blueness.  This is a view that has been presented in many forms, notable by Dretske, Lycan, and Tye.  The basic claim they all share in common is “necessarily, experiences that are alike in their representational contents are alike in their phenomenal character” (10).  This assertion, is a weaker one than the identity relation that has been postulated elsewhere, as it is merely about supervenience (defined as a relation between two sets of properties, in which, for there to be a change in one there must also be a change in the other (11)).
        Many rebuttals to this argument have been made so far, notably those involving inverted qualia and absent qualia.  In Shoemaker’s inverted qualia argument, he postulates that what one person sees as blue, another might see as red.  Obviously, there could be no objective way to determine that my red is qualitatively identical to yours, and it may well be the case that our two mental states associated with redness have the same representational content, but have quite the opposite phenomenal content.  With absent qualia, we hypothesize that mindless zombies exist, and that they have representational states, but lack any phenomenal content whatsoever.
        Wager gives a new response, that of extra qualia.  The appeal of this account is that it does not force us to engage in fantastic hypothetical examples, but is based on cases that exist in the real world.  Wager argues that, as has been observed in synesthetes, two people who are alike in terms of representational content are able to have partially different phenomenal contents.  This presents a problem “for versions of representationism that accept causal tracking or certain types of teleological accounts of intentional content” (p. 276).  Wager claims that representationists must accept that a synesthete’s qualia are phenomenally different, unless they wish to deny the very existence of synesthesia. He also briefly alludes to a possible case similar to that of inverted qualia, in which the inversion is intermodal rather than intramodal.  This might result, for instance, from a colored-hearing synesthete who sustains damage to the auditory cortex, and can only perceive sounds visually (9).
        Another, more basic problem that arises with synesthesia is that it causes us to question our deep-seated notion that there exists an objective reality.  Though we do consider possibilities such as inverted spectra, it is as little more than science-fiction examples, as most of us would agree that there is probably a strong correlation in how we perceive the world.  In all likelihood, we do so because we think that beneath it all, there are root causes that drive our senses.  The existence of these extra qualia, and in particular, their lack of agreement, seems to cast doubt on that assumption.  If one person says the letter A is red, and another says it is blue, is this because of a difference in the letter-color correspondence, or are these systems internally consistent, and it is their eyes that process color differently?  If it is the latter, what does this say about more dramatic differences in sensation, such as Michael’s shape-associated tactile responses?  Perhaps we are all functioning in completely different perceptual worlds.  If so, we might never have known because our qualia can only be expressed in their own in terms, except in a few individuals who have qualia that “speak more than one language,” as it were.