Is Solipsism Logically Possible?
It has been commonly assumed that solipsism is logically or metaphysically possible. I could exist without anything else existing. There are possible worlds in which I exist and nothing else does. I can imagine myself completely alone. Seductive as such thoughts may appear, I think they are mistaken; they arise from a confusion of metaphysical and epistemic possibility.
Suppose someone claims that this table in front of me could exist in splendid isolation, the sole occupant of an ontologically impoverished world—no chairs, planets, people, birds, etc. Well, that seems true—those absences are logically possible. But what about the piece of wood the table is made of? This table is made of that piece of wood in every possible world in which it exists, so the table cannot exist without the piece of wood. But that piece of wood came from a particular tree—it could not have come from any other tree. So this table can only exist in a world that also contains the tree in question, since it was a part of that tree. The table and the tree are distinct existences, so the table cannot exist without something else existing—the tree that donated the part that composes it. The table is necessarily composed of that piece of wood and that piece of wood necessarily derives from a particular tree: there are necessities linking the table with another object, viz. the tree. Thus “solipsism” with respect to this table is not logically possible.
Now consider a person, say me. I could not exist without my parents existing, since no person could be thisindividual and not be born to my parents. This is the necessity of origin as applied to persons. In any world in which I exist my parents exist; more precisely, in any world in which I exist a particular sperm and egg exist (and they can exist only because of the human organisms that produced them). So my existence implies the existence of my parents. Therefore solipsism is not logically possible. But the existential ramifications go further: my parents cannot exist in a world in which their parents don’t exist. And so on back down the ancestral line, till we get to the origin of life: no later organism can exist without the procreative organisms in its ancestral line. Every organism has an origin, and that origin is essential to its identity. But it goes even further, because the very first organism must have had its own inorganic origin, presumably in a clump of molecules, and that origin is essential to it—itcould not exist without that clump existing. And that clump of molecules also had an origin, possibly in element-forming stars; so it couldn’t exist without the physical entities that gave rise to it. And those physical entities go back to the big bang, originating in some sort of super-hot plasma. So I (this person) could not exist unless the whole chain existed, up to and including certain components of the big bang. Colin McGinn could not exist without millions and millions of other things existing, granted the necessity of origin. I am linked by hard necessity to an enormous sequence of distinct particulars. I couldn’t be me without them.
Of course, there could be someone just like me that exists in the absence of my specific generative sequence—though he too will necessarily carry his own generative sequence. Perhaps in some remote possible world this counterpart of mine arises not by procreation but by instantaneous generation—say, by lightning rearranging the molecules in a swamp. But even then that individual would not be able to exist without his particular origins—his collection of swampy molecules and that magical bolt of lightning. Solipsism will not be logically possible even for him. In any case, the question is irrelevant to whether I could exist without my generative sequence: my counterparts are not identical to me. All we are claiming is that solipsism is logically impossible so far as I am concerned—this specific human being. It is my existence that logically (metaphysically) requires the existence of other things—lots of other things. I (Colin McGinn) could never exist in another possible world and peer out over it to find nothing but myself (at least throughout history–I might exist without any other organism existing at the same time as me, my parents both being dead). The same applies to any person with the kind of origin I have, i.e. all human beings.
Why do we feel resistance to these crushingly banal points? I think it is in part because we confuse a metaphysical question with an epistemological question; and we cannot answer the epistemological question by appealing to our answer to the metaphysical question. The epistemological question is whether I can now provethat solipsism is false: can I establish that I am not alone in the universe? In particular, can I establish that my parents really exist (or existed)? Maybe they are just figments of my imagination; maybe I was conceived by lightning and swamp. I cannot be certain that I was not. I cannot even be certain that I have a body. I can establish that I think and exist, but I cannot get beyond that in the quest for certainty. So the existence of my parents is not an epistemic necessity. If I could prove that I am a member of a particular biological species, then maybe I could prove that I must have arisen by sexual reproduction from other members of that species: but the skeptic is not going to let that by–she will demand that I demonstrate that I am a particular kind of organism arising by sexual reproduction. And I will not be able to meet that challenge, since there are conceivable alternatives to it (the hand of God, swamp and lightning, the dream hypothesis). Maybe I just imagine that I am a biological entity with parents and an evolutionary history. So we cannot disprove solipsism in the epistemological sense: for all I know, there is nothing in the universe apart from me.
But this is perfectly compatible with the thesis that it is not in fact logically possible for me to exist without other entities existing along with me: for if I am a biological entity born by procreation, then my existence logically implies the existence of many other things. It is just that I cannot prove to the skeptic’s satisfaction (or my own) that that is what I am. I might come to the conclusion that I had no parents after all, but that will not make it the case that there are metaphysically possible worlds in which I had no parents—this is a matter of the facts about me, not my beliefs about the facts. Thus solipsism is an epistemic possibility but not a metaphysical possibility. It is just like the table being both necessarily made of wood (metaphysical) and also being possibly not made of wood (epistemic). Given that I arose from biological parents, I necessarily did; but it is an epistemic possibility that I did not so arise—I could be mistaken about this.
It would be nice to disprove solipsism, but it isn’t insignificant to show that it is not in fact logically possible, given the actual nature of persons. Persons are the kind of thing that implies the existence of other things (granted that we are right in our commonsense view of what a person is). In this they resemble many ordinary biological and physical entities, which also have non-contingent origins. We may feel ourselves to be removed from the world that surrounds us, as if we are self-standing individuals, ontologically autonomous—as if our essential nature could subsist alone in the world. But that is a mistake—we are more dependent on other things than we are prone to suppose. We are more enmeshed in what lies outside of us than we imagine. We suffer from illusions of transcendence and autonomy. We are not free-floating egos that owe no allegiance to anything else; we are essentially relational beings, our identity bound up in our history. We cannot be metaphysically detached from our origins, proximate and remote.
The same point applies to our mental states: they too cannot be separated from other things. Could this pain exist in complete isolation? That may seem like a logical possibility, but on reflection it is not: first, this pain’s identity depends on its bearer—it could not be this pain unless it had that bearer; and second, the identity of the bearer depends on the kind of history it has. So this pain could not exist without the generative sequence that gave rise to its bearer, a particular living organism; and that depends upon billions of years of history, going back to the big bang (and before). There is no possible world in which this pain exists and certain remote physical occurrences don’t exist. There are necessary links connecting present mental states with remote physical occurrences—from the joining of a particular sperm and egg, to the origin of mammals, to the production of chemical elements. My pains can’t exist in a world without me (you can’t have my pains), but I can’t exist in a world without my parents, and my parents can’t exist in a world without their remote primate ancestors, and these ancestors too had their own necessary origins. The pains that now occur on planet Earth (those pains) could not exist in a possible world without an elaborate biological and physical history that coincides with their actual history.
It is an interesting fact that we recognize these necessities. On the one hand, we have quite strongly Cartesian intuitions about the person and the mind, which is why dualism and solipsism appeal to us—these seem like logical possibilities. But on the other hand, we are willing to accept that the person and mind are tied to other entities with bonds of necessity—as with the necessity of personal origin. We recognize that the identity of a person cannot be radically detached from all extrinsic and bodily things—parents, sperms, and eggs. These are anti-Cartesian intuitions insofar as they dispute the self-subsistence of the self.  We are thus both Cartesian and anti-Cartesian in our modal instincts about persons. It is as if we know quite well that the self cannot be a self-subsistent non-material substance without logical ties to anything beyond itself, even though in certain moods we fall prey to such thoughts. We know that our essence implies the existence of other things—as demonstrated by the necessity of origin—and therefore solipsism is not in fact logically possible. We are modally ambivalent about self and mind, but not confused.
 Kripke mentions the anti-Cartesian consequences of the necessity of origin at the very end of Naming and Necessity (footnote 77, p. 155). What is surprising is that neither he nor anyone else seems to have noticed the consequences for solipsism (including myself, and I published an article on the necessity of origin in 1976). But it is really just a fairly obvious deduction from the necessity of origin (originally proposed by Sprigge in 1962, as Kripke notes).