Usually we think of mental acts as private. The contents of my consciousness are available to me and no one else. Thus we tend to describe these experiences as private, since they almost define what privacy is, and not accidental privacy, but essential privacy; others do not have my conscious thoughts. Some think that because mental facts are private they must be essentially separated from the public facts, thus that privacy supports a kind of dualism.
Of course if we are materialists, or at least monists, we must reject privacy as a defining feature of mental events, since ultimately everything must be reduced to facts that are public in a monist scheme (well, unless we are solipsists). Often dualist positions are supported by conceptual analysis style arguments, which conclude that it is part of the essence of mental acts that they be separate from the physical. But we can’t claim that they are essentially private for this reason, because the idea of ESP, someone else sharing your supposedly private thoughts, is certainly conceivable. Many even believe it to be possible. So a reliance on conceptual analysis, often put forward as an argument for dualism, is out of the question in this case.
If one hopes to defend the idea that there really are such private facts the best defense is to claim that privacy, as we experience it, can’t be explained by a monist theory. This is not a very good defense, however, since such an explanation is not that hard to come by. Most explanations of privacy turn on the fact that even though all of the physical information is theoretically available to us we don’t actually have access to it all. For example, I don’t know anything about planets orbiting stars in distant galaxies. This doesn’t make this information of an essentially different kind then the normal physical information, however, it simply stems from the fact that what we know about is constrained by our ability to observe what exists. And just because we can’t currently observe something doesn’t mean that it is of a different nature, we are not verificationists. Thus we can explain the apparent privacy of the mind by comparing it to a black box. A black box, in this context, is something for which we don’t yet have the tools to make observations about its internal states. Of course, some may object, saying that we do know how to look at the internal state of the brain. This is true, but we don’t yet have to tools to interpret that information meaningfully (in terms of consciousness and experience), which is essentially the same thing as not being able to observe it at all in terms of information. Even though we can observe a black box’s behavior we can never be completely certain about its internal states (there are always multiple explanations for any sequence of behavior, especially complex behavior), and thus until we can look into the back box its internal states are essentially “private”, but not because there is something special about them that prevents them from being public in principle.
But, the objectors will continue, even if we could look inside such a black box we aren’t being presented with the information about the box’s internal states in the same way the box knows about them (let us assume this is a conscious box). So even if we know the same facts we don’t know about their mode of presentation, and thus this mode of presentation is essentially private. Let’s just assume for the moment that this is the case, that we can’t be presented with information in the same way as the box. Why does this mean that we can’t know about the mode of presentation? We can know how knowledge of its private facts will be incorporated into the operation of the box, and its future internal states. Besides this what more is there to know? Again, some might say the “feel”, but what can the feel be besides some fact about how the information is handled by the box? If it is something besides this we could construct boxes that operated in the exact same way, both in internal and external states, but without the “feel”. But the boxes without the feel would still think they had the feel, all their internal states would be the same. Even without the feel they would behave and think in the same way (if they had thoughts). They would even be convinced that they had this feel, since conviction is some form of thought. And this is ridiculous. We only assert that the feel exists because we have experienced it, which really only shows that we think we have the feel. But even if we didn’t have the feel we would still think in the same way, and so there is no way of knowing that we really have the feel at all. Every experience we have that has a specific feel could simply be a case of us thinking it had the feel, with no feel at all. Thus it makes much more sense to identify the feel with something about the way the information is incorporated into the system. Then we could really know that things feel a certain way to us, because of some way they are presented to us, or some way we think about them, but the feel would no longer be private.
Of course this argument doesn’t defeat dualism, it wasn’t meant to. It is possible, although unlikely, that the mind is a black box that is constructed with a different kind of substance. However, it does show that privacy can’t be used as an argument against monism (and my favorite kind, materialism).