The skeptic characteristically maintains that we have a tendency to believe in too many things. We believe in other minds (not just our own) and we believe in external objects existing independently of our mental states. Strictly, we should believe in our own mind and nothing else. There are (or there may be) fewer things in heaven and earth than are dreamt of in our philosophy. Solipsism-of-the-moment is the only safe position, which cuts the world down to one thing. Our problem is that we overestimate the contents of reality, postulating things in which we have no right to believe. We are like polytheists who should really be monotheists, or theists who should really be agnostics or atheists. We are prone to “false positives”—assuming things to exist that we have no good reason to believe exist. Skepticism thus seeks to reduce our range of beliefs in things—we must subtract, eliminate, deny.
But there is another form of skepticism, which is far less familiar: this kind says that we have a tendency to underestimate reality. We tend to assume that there are not various things, but there may in fact be such things. There may be more things in reality than we are inclined to suppose (hence Hamlet’s famous line). I call this “plurality skepticism”. The clearest example is plurality skepticism about our knowledge of other minds: not only may there be minds other than our own in the form of human and animal minds, there may also be plant minds, bacteria minds, and even molecule minds. That is, this is the skeptical hypothesis we need to rule out if we are to maintain our usual restrictive ontology of minds; and since it cannot be ruled out, the plurality hypothesis may be true. The radical plurality skeptic about other minds insists that we cannot rule out the hypothesis that there are other minds everywhere: in our own bodies, in trees, even in atoms. This skeptic will point out that the lack of behavior indicating the presence of a mind does not logically entail that there is no mind present, so we cannot cite the lack of behavioral evidence as proving that the hypothesis of a plurality of minds is false. For all we know, there are vastly more minds than we suppose. That is, we are guilty of “false negatives”—disbelieving in things that actually exist. The skeptic is not saying that we should believe in these things, only that we have no right to rule them out—so we should be agnostic.
Maximum strength skepticism says that we should both doubt the existence of minds where we typically don’t doubt them and doubt the absence of mind where we typically assume such absence. Indeed, it may be that other human beings and animals have no minds but fungi (and only fungi) have minds: this is what the consistent radical skeptic about other minds will contend—that we may be completely wrong in our habitual assumptions about the distribution of minds in nature. We may be guilty of false positives and false negatives. How can you rule out the hypothesis that only fungi and you have a mind? Are you certain that this is not the case?
We find the same structure with respect to skepticism about our knowledge of the external world: might there not be many worlds that we fail to recognize? The traditional skeptic argues that we are rash to believe in a world outside of our own mind, but the plurality skeptic argues that we are rash to assume that there is only onesuch world—the one that seems to us to exist. Maybe there are many external worlds that we fail to recognize (as some physicists today actually maintain)—maybe we vastly underestimate the content of physical reality. There is not just the ordinary world of tables, chairs, mountains, and galaxies, but also other types of object of which we have no knowledge—objects completely hidden from us (“dark universes”). Indeed, it may be that our ordinary external world does not exist (we are brains in a vat) but that other external worlds do exist: what we experience is an illusion, but there are other strange worlds out there that exist instead. That is the truly radical form of external world skepticism: it may be that the world we think we know does not exist but that other worlds do exist. These worlds are not merely possible worlds, but actual worlds that we fail to experience. The skeptic wants to know why we don’t take this possibility more seriously. More strongly, he claims that his skeptical hypotheses are as likely to be true as the view we habitually accept. He thinks we have a dogmatically narrow conception of reality. There may be all sorts of objects out there that we fail to recognize, and it may be that those in which we believe fail to exist at all. Our epistemic failings are thus multiple and grievous, not limited to postulating one world that may not exist. We should be agnostic about the full range of possibilities.
The plurality skeptic may also point out that we have a history of underestimating reality: we tend to assume there are fewer minds and physical objects than there really are. Sometimes we overestimate, as with witches and gods, but more often we underestimate. We used to restrict minds to humans, with one mind each, and even doubted the existence of minds in humans we found alien; then we acknowledged minds in other animals and all humans, as well as accepting unconscious minds within ourselves. We have grown steadily more expansive with the concept of mind. Likewise, we once limited the external world to objects perceptible by the senses, only gradually accepting microscopic objects (organisms and particles) and remote celestial objects, and now invisible forces, dark matter, and so on. We erred on the side of epistemic stinginess, which is not surprising given our limited senses, with occasional lapses into excessive ontological largesse. So the plurality skeptic can reasonably suggest that we might still be committing false negatives: that there are likely to be many more minds and external worlds than we customarily suppose. Our confidence that we have accounted for all realities might well be misplaced. We may be committing more errors of exclusion than of inclusion. At any rate, radical skeptical hypotheses postulating pluralities of minds and worlds cannot be ruled out.
The skeptical problems of the external world and other minds are generally presented as problems of epistemic over-commitment, with the emphasis placed on the possibility of false positives—there might be less to reality than we suppose. But it is equally important, in presenting the full skeptical challenge, to reckon with the problem of false negatives—the possibility that there might be much more to reality than we suppose. We don’t know if there is no external world or many (perhaps infinitely many) external worlds or just the one world that we normally take for granted; and we don’t know if there is just one mind (one’s own) or hugely many (perhaps infinitely many) minds, spread everywhere and in the most unlikely places, or just the range of minds we normally take for granted. We are really very ignorant of what is so and what is not so—that is the skeptic’s message in its full generality. We may be making enormous errors of both commission and omission. We might live in a very lonely world or in a very crowded world—we simply cannot know.