The solipsist is a partial skeptic: he doubts the existence of other beings, sentient or otherwise, but he doesn’t doubt his own existence. He might go on positively to affirm that nothing exists but himself. He favors one object over all others. What is the analogue of this for the material world? Suppose I come to the conclusion that no material object exists except my coffee cup: it is the sole object that is real, all the rest being illusory. I am a kind of coffee cup solipsist—I favor one material object above all others. Perhaps I feel that I know my coffee cup (hereafter “the Cup”) better than any other material object, given my close daily acquaintance with it; in any case I select this object as the sole possessor of reality (apart from myself). I may think I am a brain in a vat of the classic type, but with one exception—the Cup. Just as the traditional solipsist picks out one individual as real (himself), so this “solipsist” picks out one object as real, namely a certain cup. Evidently this is a coherent position: it is an epistemic possibility that only the Cup exists among all the other apparent material objects. Perhaps the mad vat scientist has set things up so that the only veridical perceptions are of the Cup, the rest being hallucinations; or the architects of the Matrix have a peculiar cup fixation. Everything in my visual field is an illusion save for that solitary beverage container. Hey, it’s logically possible. It is a question whether this form of partial skepticism is a new type of skepticism: is it another skeptical scenario that needs to be considered? Apparently it is. And once we have this type of skepticism on the table we can construct varieties of it: for example, are oak trees the only real trees? We just select a class of material objects as exempt from general skepticism. Similarly, a partial solipsist skeptic could, logically, claim that only persons of a certain type exist—say, only people under six feet tall. That again is an epistemic possibility.
Why do these partial skeptical positions seem completely arbitrary and unmotivated? Formally they resemble classic solipsism, but they seem entirely without intuitive appeal. The reason is obvious: they have no epistemological basis. There is no reason to favor a single coffee cup or a particular species of tree or a certain height of person. But there is a reason to favor myself over all other objects, namely my immediate infallible knowledge of myself: I know for certain that I exist but not that other people exist. By contrast, I have no greater certainty about the Cup than about other objects: so it seems completely arbitrary to favor that object above all others. I wish to make two points about this observation. The first is that there could be epistemological reasons to favor the Cup: that is, there are possible worlds in which there is more reason to believe in the Cup than in other material objects (irrespective of whether either really exists). Suppose I have regular, clear, and vivid perceptions of Cup (it has a name now) but irregular, unclear, hazy perceptions of other objects—so much so that the possibility of hallucination becomes highly plausible for them. Then I would have reason to think that Cup exists but other objects probably don’t—whether or not this supposition is actually correct. If so, I might be tempted to accept a form of partial skepticism about the material world, or even to deny that any material thing exists apart from my beloved Cup. That is, in this world a position analogous to traditional solipsism would be rational (assuming that solipsism is rational): there is more reason to believe in one material object than in any others, as there is more reason to believe in myself than in other selves.
The second point is that solipsism as a positive doctrine starts to look distinctly irrational in the light the analogy. For why should we entertain the move from an epistemological point to an ontological one? It is true enough that Cup might be the only existent material object—this is an epistemic possibility—but why should this incline one in the slightest to suppose that it is the only existent object? Likewise, it is an epistemic possibility that only I exist—this cannot be ruled out with absolute certainty—but it is a total non sequitur to infer that it is actually true. Such a conclusion is as arbitrary as supposing that only my coffee cup exists, even when I have more reason to believe that it exists than to believe in the existence of other material objects. The belief in solipsism is as irrational as the belief that only precious Cup exists: both are logical possibilities, to be sure, but there is not even a smidgen of evidence to indicate that either supposition is true. What is true is that people care a lot more about themselves than about other people, whereas they don’t tend to have such strong feelings about their coffee cups; but that is not a reason positively to believe that you exist and other people don’t. Certainty is one thing, truth another. Solipsism is no more likely to be true than the corresponding belief about a particular cup. Neither position can be refuted as an epistemic possibility, but neither position has anything solid in its favor. Yet people have been more strongly drawn to solipsism than to the analogous position with respect to material objects. Why? 
 I have not discussed here alleged reasons for supposing that only the concept of one’s own mind makes sensegiven the nature of mental concepts (only first-person uses are properly intelligible), limiting myself to the more popular claim that epistemological reasons favor solipsism. The idea that one’s own mind is uniquely favored ontologically should strike us as remarkably self-centered, a reflection perhaps of our natural selfishness.