A Private World
We live in a mixed world: partly public, partly private. There are public perceptible states of affairs and there are private imperceptible states of affairs—for example, rocks and animals, on the one hand, and thoughts and sensations, on the other. You can see the color of your cat’s eyes but you can’t see what she is thinking and feeling. Her inner mental life is private while her outer bodily features are public. The mind escapes sensory observation while the body is open to sensory observation. This is a common, indeed platitudinous, statement: you can’t gaze at the mind in the way you can gaze at the brain; you can’t look at the mind; you can’t take a magnifier to it. I won’t defend this platitude here, but I will have to say something by way of interpretation—for the notion of privacy is not pellucid. Some might suggest that the notion comes down to an epistemic asymmetry between the subject and outside observers: the subject of the mental state knows about it directly while other people only know about it (if at all) by inference. But this neglects the case in which the subject has no knowledge of his inner states—say, a cat with no introspective faculty. In such a case the subject has a private mental state but not because he has superior knowledge of the nature of that state, since he has no such knowledge. Rather, the mental state is a private entity in its own right, not because of an alleged epistemic asymmetry: it is intrinsically imperceptible by others, whether known about by the subject or not. Also, we cannot equate the notion of privacy with the notion of imperceptibility, since atoms are imperceptible but not private in the intended sense: atoms are the kind of thing that could be perceived by the human senses, but mental states are incapable of such perception. They are hidden in a special way, not in the way microorganisms and atoms are hidden (these are public entities). So imperceptibility is necessary for privacy but not sufficient. The difficulty comes in spelling out what this special notion of hiddenness comes to, though we have a strong intuitive sense of it. But I will not consider the question further, being in pursuit of other quarry. Let it suffice to say that some facts in the actual world are public and some are private. The former are typically “physical” and the latter are typically “mental”. But note that the concept of the private does not entail that everything private is necessarily mental: there could perhaps be non-mental things that are private in the intended sense—hidden in that special way that mental things are hidden. I know of no actual examples of such things, but the possibility is not analytically ruled out by the concept of privacy. The best way to understand the philosophical concept of the private is simply as the non-public; then we can say that our world is partly public and partly non-public. It has a dual ontology.
Now I can ask my question: are they any possible worlds that are completely public or completely private? We might think the answer to the first question is obvious: of course there are—any worlds without minds in them. That indeed sounds right, but we must not forget our panpsychist friends—they hold that matter in general has a mental aspect (and may in fact be completely mental). If that were so, then all worlds with our kind of matter in them would be worlds with a private dimension. Perhaps there are worlds containing matter that has no mental dimension at all, in which case these would be completely public worlds; but that is a substantive metaphysical claim and not beyond serious controversy. If panpsychism were true of our world, it might be that most of the actual world is private, if the hidden psychic dimension were rich enough (compare the proposition that most of matter is dark matter). But the really interesting question is whether a world could be completely private, i.e. contain no public facts at all. Again, this is not the same question as whether a world could be completely mental, since privacy might take non-mental forms; it is the question of whether any type of private fact, mental or non-mental, could exist in the complete absence of public facts. That is, could there be a purely private world? If you were to travel there in your inter-world space ship, you wouldn’t see a damn thing, because everything would be hidden from view in that special way that mental states are hidden. This world will appear to contain nothing, though it could be that all sorts of perturbations are going on in the private realm that constitutes it—perturbations with no public face. This is a world of pure unadulterated privacy: everything that exists in it is invisible, like thoughts and sensations. Is such a world really possible?
First, it might be wondered whether the private can exist without a bedrock of the public: in our world it seems that there can be no private fact without an underlying public fact (minds require brains). Second, don’t we need space and time, and aren’t these public entities? That question isn’t easy to answer because space and time might themselves be private entities—after all, we don’t perceive them with our senses. However, I think we can confidently state that a possible world could be mainly private, with only a smattering of the public at its periphery; and that is the point of philosophical interest. There is no necessity that the public must dominate across logical space: for all we know, possible worlds dominated by the private are commonplace. In any case, a world could be largely private—full of “private matter” (“matter” in the sense of “stuff”). I suppose it might even turn out that ourworld is largely private (consider panpsychism again), if reality contains a lot of hidden non-public aspects. Privacy might be the rule not the exception, in our world and across possible worlds. The appearance of preponderant publicity might be an illusion born of our local cosmic condition and our limited senses. Privacy might be the basic form that reality takes.
I now want to ask a different modal question: are privacy and publicity essential or contingent properties of what has them? Thoughts are private in the actual world, but are they private in all possible worlds? Bodies are public in the actual world, but are they public in all possible worlds? One’s first reaction to these heady questions is apt to be that these are not real possibilities: privacy and publicity are essential properties of what has them. But the question is not as easy to answer as might be supposed, because the properties at issue depend on the existence of certain sorts of sense organ. Is it not logically possible for there to be a sense that can respond directly to the thoughts of others? Couldn’t a sense organ be causally linked to other brains in such a way that an impression of a thought is produced in the mind of an observer? Isn’t telepathy at least a logical possibility?  And can’t we imagine a possible world in which bodies are hidden from perceiving creatures because of the absence of the requisite sensory apparatus? These creatures can see another creature’s states of mind but they don’t have any of the normal human senses, and so cannot form sense impressions of ordinary physical objects. Admittedly, these are far-out logical possibilities, and we may not be able even to adequately grasp what they involve, but they seem describable and coherent. We could have public-private inversion in a possible world: minds public, bodies private (non-public). It all depends on what kinds of sense organs exist in the world in question. People might believe in material objects and even know of their existence, but nothing gives them the sensory impression of such objects; and they may be assailed with impressions of other minds, as we are assailed with impressions of other bodies. Logical space is large and accommodating. In our world, with its specific laws and sense organs, thoughts are necessarily private and bodies are necessarily public; but if we reach out to the outer limits of possibility we may find undreamt of possibilities—such as creatures with eyes for minds but no sensory access to ordinary physical objects. Accordingly, there might be worlds that are completely mental yet completely public, as well as worlds that are completely physical yet completely private. So the completely private worlds I mentioned earlier need not be mental worlds at all: they could be worlds consisting of facts unlike any in our world, or they could simply contain ordinary physical facts. How things are in our world is a poor guide to the full extent of modal reality; our world might be quite unrepresentative of logical space. In our world privacy is the exception not the rule, and it is linked closely to the mind; but both of these truths are fungible when we consider the full range of possibilities. The concepts of privacy and publicity need further examination; in particular, we should treat privacy as a natural fact about the universe that needs to be investigated, empirically and conceptually. Here I have tried to open up the subject of its modal status. 
 One difficulty here is that it is not easy to say why thoughts and sensations are private: what is it about them that makes the private? Are they just private as a brute fact with no rationale that can be provided? Or is their privacy a consequence of their subjectivity or their intentionality or some other feature? But why exactly should these properties give rise to the property of privacy? It is hard to see what explains the privacy of the mental, or whether any such explanation should be sought. Privacy is obscure, possibly terminally mysterious.
 We can formulate parallel questions about subjectivity, i.e. what-it-is-likeness. Can there be a completely subjective world, or a completely objective world? Is subjectivity an essential property of what has it (likewise for objectivity)? The relationship between subjectivity and privacy is also worth investigating, but deeply obscure—like everything else in this area.