There are a number of importantly different views associated with the term solipsism. Its Latin roots—solus, meaning "alone," and ipse, meaning "self"—suggest the rough idea that a solipsistic doctrine is going to put some sort of emphasis on the self standing alone, but there are radically different ways in which a philosopher might develop that emphasis. In particular, we must distinguish an extreme metaphysical thesis, a view about the nature of mental states (sometimes misleadingly referred to as methodological solipsism), an epistemological/methodological thesis, and an ethical thesis.
The simplest and most radical of doctrines associated with solipsism is the puzzling doctrine that only the self exists. Stated in these terms, the doctrine is scarcely intelligible. The obvious question concerns whose self precisely it is that is supposed to be the only existing thing. It is easiest to state the doctrine from the first-person perspective. If I embrace solipsism, I am endorsing the view that I am the only existing thing. If you embrace solipsism, then you are endorsing the view that you are the only existing thing. If we both endorse solipsism, therefore, then we are both wrong. In asserting solipsism, the solipsist is usually not trying to deny the existence of properties exemplified by the self. So the self that exists may believe, fear, hope, plan, and so on. We can also distinguish the solipsist who intends only to deny the existence of other minds from the solipsist who denies the existence of all other objects, for example, physical objects. It would be odd, however, to hold the former without the latter for, as we shall see, the epistemological position that drives one to a skepticism about other selves often involves a skepticism with respect to the external world.
There is almost a comical aspect to the most extreme form of solipsism. It is certainly odd to hear any philosopher defending (to whom?) the view. One could certainly never take comfort in the fact that one succeeded in convincing anyone of the truth of the view. But in this respect solipsism is probably no worse off than any other extreme form of skepticism—say skepticism with respect to the past, the future, or the external world. In fact, solipsism is probably a view that one starts to take seriously precisely in the context of more general epistemological concerns. So, for example, while Descartes was no solipsist, he came perilously close to painting himself into a solipsistic corner.
In the Meditations, Descartes famously sought secure foundations for knowledge. To find those foundations he employed what is sometimes called the method of doubt. He tried to strip from his belief system all those beliefs that admit of the possibility of error. So, for example, he thought that no belief about the physical world belongs in the foundations of knowledge because our evidence for believing what we do about that world never gets any better than vivid sense experience. But the kind of sense experience upon which we must rely is always compatible with our dreaming, or our being the victims of massive demon-induced hallucination. Since our knowledge of the existence of other people seems to rest critically on our knowledge of other bodies, skepticism with respect to the physical world might seem to entail a skepticism with respect to the existence of other selves. After rejecting a number of candidates for foundational truth, Descartes finally hit upon his own existence as one truth that he could not rationally doubt. No matter how hard he tried to convince himself that he did not exist, such efforts merely reinforced for him the fact that he did exist. One can only doubt one's own existence if one exists to do the doubting. "Cogito, ergo sum," Descartes concluded—I think, therefore I am.
While the exact nature of the evidence or justification to which Descartes appeals in claiming foundational knowledge of his own existence is a matter of some controversy, his attempt to begin a reconstruction of the rest of what he knows from this foundation is one that could have easily led him to a solipsistic conclusion. Descartes thought that he could find a way of legitimately inferring the rest of what he believes from knowledge of his own thoughts and experiences, but it is an understatement to suggest that his efforts did not meet with universal acceptance. Indeed, many contemporary philosophers are convinced that if we restrict ourselves to premises describing our own existence and the conscious states exemplified there, there is no path to knowledge of, or even justified belief in, the rest of what we commonsensically think we know.
The kind of radical foundationalism that Descartes embraced might naturally lead, then, to the conclusion that we can only know of our own existence and the perceptions and thoughts that reside there. And if one restricts one's metaphysical positions to what is licensed by knowledge, then one might be left affirming only one's own existence. Again, that claim is usually expanded, even by the solipsist, to include the conscious mental states exemplified by that self. When we discuss epistemological solipsism, we will say more about the epistemological assumptions that might lead one to take seriously the position of metaphysical solipsism. But let us first examine some influential criticisms leveled at the view.
One charge often leveled against metaphysical solipsism is the charge of self-refutation. There are a number of different ways in which a view might be self-refuting. The strongest form of self-refutation is logical—a self-refuting view entails that it is itself false. So, for example, the proposition that all claims are false is self-refuting in this sense. The claim entails its own falsehood. On the face of it, it is difficult to see how the solipsist's claim can be self-refuting in this way. Nevertheless, critics have claimed that for the solipsist's claim to be meaningful it must be false. Inspired by Wittgenstein, for example, some philosophers claim that language and meaning are essentially social; there can be no such thing as a private language or a private linguist.
Unfortunately, it is by no means easy to figure out just what the basis for this claim is. One crude characterization of the argument emphasizes the importance of rules in determining meaning. One uses a term meaningfully only if one uses it in accord with a rule that determines when the term is used correctly or incorrectly. If one is the sole arbiter of when a term is used correctly, the argument goes, one will be unable to make a mistake. But if one cannot make a mistake using the term, then it makes no sense to suppose that one is using the term correctly; correct use makes sense only against the possibility of incorrect use. It is only when there is a community of language users that one can understand the distinction between correct and incorrect use of language; incorrect use can be identified with divergence from standard or common use.
So to illustrate with an example, suppose that I see a creature I have never seen before and resolve to call it and anything relevantly like it a "gretl." One might initially suppose that I have successfully introduced a word into my own private language. Tomorrow, I see another creature—somewhat like the first, but also in many ways dissimilar. Is it a "gretl" or not? It seems that if I am the only one deciding whether it is enough like the first creature to count as a "gretl," then I cannot get it wrong—whatever I decide goes. Again, the Wittgensteinian will claim that where there is no possibility of error, there is no possibility of truth.
There are no uncontroversial interpretations of the private language argument, and a full evaluation of it would take us far afield. All versions of the argument, however, rest on highly controversial assumptions. It is not clear, for example, that judgment involves comparison to a paradigm. In any event, one must surely worry that the version stated above would rule out even the possibility of a solitary linguist—a sole language user. But it is hard to see how it could be impossible for there to exist one and only one person who was capable of both thought and language. We can imagine, for example, an infant who is the sole human survivor of a worldwide natural disaster and who, adopted by apes, somehow manages to mature into an adult. In such a world, if that human being could formulate the thought that there are no other people, he or she would have formulated a true thought. And do we really want to argue that in the situation described it would be metaphysically impossible for the person to formulate either the thought or a language that could express the thought? If we reflect on the scenario just described, we might become suspicious of any argument that purports to show that the solipsist's doctrine that there exists only one self is in some sense unintelligible or necessarily false.
There is a more recent philosophical claim about the nature of thought that, like the earlier arguments inspired by Wittgenstein, might seek to cast doubt on the intelligibility of metaphysical solipsism. It is sometimes called semantic, psychological, or content externalism. The basic idea behind the view is encapsulated in Putnam's famous slogan that meanings are not "in the head," and its proponents sometimes seem to claim that one can only have thoughts about certain kinds of things if those kinds of things exist. If the view were true, one might be able to infer from the fact that one can form thoughts about physical objects (even the thought that there are no physical objects) that physical objects exist. Similarly, one might be able to infer from the fact that one can form thoughts about other people (even the thought that there are no other people) that other people exist. The view underlying this criticism of metaphysical solipsism is held in opposition to another thesis associated with solipsism, a thesis sometimes called "methodological solipsism."
The term methodological solipsism was introduced by Hilary Putnam and made more familiar by Jerry Fodor. It is precisely the view rejected by the content externalist. The methodological solipsist (or internalist in the philosophy of mind) is convinced that psychological states (beliefs, desires, fears, pains, etc.) are entirely constituted by internal features of the person in those states. Two people cannot be in identical internal states while one of them has a certain desire, say, and the other does not. The externalist argues, somewhat paradoxically perhaps, that at least some of the conditions that constitute or determine your psychological states are factors that lie outside you—factors that include, for example, the causal origin of your internal states. So Putnam famously argued that two people could be in precisely the same internal states while one is thinking about water (the stuff with molecular structure H2O) and the other is thinking about "twater" (something with an entirely different molecular structure). The difference in the content of their thoughts would be a function of the environments in which the respective internal states arise. In a much-discussed attempt to extend these considerations to issues involving skepticism, Putnam (1981) appeared to argue that if one were a brain in a vat whose experiences were produced by the machinations of some mad neurophysiologist, one could not even entertain that hypothesis. His idea is that without some sort of sensory interaction with the physical world, one could not even form a thought that was about a physical object like a brain or a vat.
If such an argument were successful it would not be hard to extend it as an attack on the intelligibility of the more extreme forms of metaphysical solipsism. When the solipsists make clear their views about what does not exist, their ability to form the thought, for example the thought that there is no external world, presupposes that there is one. Without interaction with external reality, no thought could be about such reality and one thus could not even coherently deny its existence. Since skepticism about the existence of others typically runs through skepticism about the external world, one will have undercut an argument for solipsism.
Content externalism is no less controversial than the various presuppositions Wittgenstein and others brought to their philosophical views about meaning. But even if we grant some of the basic tenets of the externalist's conception of the conditions necessary for thought, careful statements of the view will not take one very far toward interesting metaphysical conclusions about what there is. For one thing, the careful content externalist is going to radically restrict the view to a subclass of thoughts. No one thinks, for example, that in order for one to have thoughts about mermaids, one must have interacted in some way (or be connected with someone else who has interacted in some way) with actual mermaids. The most natural move, borrowed from the earlier empiricists who thought that all ideas are "copies" of prior impressions, is to make a distinction between complex ideas and simple ideas. The earlier empiricist conceded that the idea of a mermaid is not a copy of some prior impression or experience of a mermaid, but went on to claim that the idea is complex (the idea of woman's torso combined with a fish's tail), and the ideas out of which the complex idea is composed are copies of prior impressions. Of course, the idea of a torso itself might be complex, composed of still simpler ideas. The natural thought for both the earlier empiricist and the content externalist is to restrict their thesis to the simple ideas that are the "building blocks" of other ideas.
The difficulty is that it is not clear what the best candidates are for the simple ideas out of which others are built. Suppose, for example, that I have the idea of a sensation. I might also be able to form the idea of causation. I can put those two ideas together to form the idea of that which causes the sensation. Arguably, in this way I can form the complex idea of an external object. But I have formed the idea in such a way that it might not correspond to anything—the sensation in question might have no cause. There seems to be nothing in the externalist's view that blocks the possibility of forming thoughts of this sort, thoughts that might well not correspond to anything. Consequently, it is not at all clear that the metaphysical solipsist would face any problems of self-refutation in framing various radical views about what does not exist.
The first two theses discussed above are metaphysical claims—claims about what exists. As we have just seen, one use of the expression "methodological solipsism" involves a claim about the nature of mental states. There was, however, an earlier use of the term "methodological solipsism" (by Hans Driesch, Rudolph Carnap, and others) expressing an epistemological thesis. Indeed, that earlier use of the expression is a much more natural way to describe what these philosophers had in mind—a method for arriving at truth. To avoid confusion, it is best to describe the view that I have in mind as epistemological solipsism.
The fundamental idea behind epistemological solipsism is the claim that in reaching conclusions about what exists, each of us is restricted to a foundation of knowledge about our own mental states. The foundationalist in epistemology is convinced that there must be some truths that are known or justifiably believed without their needing to be inferred from other different truths that are known or justifiably believed. This foundational knowledge is needed to block a vicious epistemological regress. To justifiably believe P by inferring it from E1, one would need, the argument goes, justification for believing E1. Some would argue that one would also need justification for believing that E1 confirms P. But if the only way to justifiably believe something is to infer it from something else, then to justifiably believe E1, one would need to infer it from something else E2, which one would need to infer from something else E3, and so on, ad infinitum. Finite minds cannot complete infinitely long chains of reasoning. It is not even clear that infinite minds can complete infinitely long chains of reasoning. So if we are to justifiably believe anything at all, some of our beliefs must be non-inferentially justified—justified without inference.
The radical empiricist/epistemological solipsist is convinced that the only contingent truths that one can know without inference are truths about one's own existence and the thoughts and experiences contained there. Arguments for restricting the foundations of knowledge in this way depend, typically, on specific presuppositions about the nature of foundational knowledge. As we saw earlier, Descartes sought foundations in beliefs that are infallible. If one's justification for believing something is compatible with the belief's being false, then the belief is not a candidate for non-inferential knowledge. The radical empiricist was convinced that beliefs about the external world, the past, other minds, and the future, all fail this test for foundational knowledge. By contrast, one's beliefs that one exists, that one is in pain (when one is), that one has thoughts, all were supposed to pass the test.
A closely related version of foundationalism seeks to identify foundational knowledge with belief accompanied by direct acquaintance with facts that are the truth-makers for the belief. On this view, when one is in pain, for example, one's non-inferential justification consists in the fact that the pain itself is directly present to consciousness. Again, the claim is that objects in the physical world, other minds, facts about the past, and facts about the future, are never directly presented to consciousness in this way. Their existence must be inferred from what is known directly about present conscious states.
The epistemological solipsist's position was probably almost taken for granted by most prominent philosophers in the history of philosophy. The task of the philosopher is essentially egocentric. If one is to avoid begging questions, one has no choice but to begin one's search for truth with the various ways that things appear. This epistemological position does not entail metaphysical solipsism, but as we saw, there is the danger that one will be unable to reason oneself out from behind this "veil" of subjective appearance.
The version of foundationalism endorsed by the epistemological solipsist has come under sustained attack in the last several decades. In discussing metaphysical solipsism, we have already had occasion to examine Wittgenstein's worries about the possibility of a private language. To the extent that judgment involves categorizing things, categorizing things involves appeal to the correctness of following certain rules, and knowledge of what rules sanction involves facts about communities of rule followers, one will have difficulty finding the kind of foundations sought by the epistemological solipsist. But as we saw, this criticism of private language and thought is by no means uncontroversial.
In the previous section, we also discussed a view about the nature of mental states that might also cast doubt on the radical version of foundationalism endorsed by many empiricists. If external reality is literally partially constitutive of mental states like belief, then it might seem to follow that our knowledge of mental states could be no more secure than the knowledge of that external reality upon which their content depends. That this follows from psychological externalism, however, is a matter of great dispute, and among those who take it to be an implication of externalism in the philosophy of mind, there are many who take this consequence of the view to be a reductio of the view. In any event, as we also noted, psychological externalism is no more uncontroversial than the presuppositions of Wittgenstein's argument against the possibility of knowing truths about a "private" experience.
There are other efforts to cast doubt on the claim that empirical knowledge begins (and perhaps even ends) with knowledge of one's inner mental states. In a famous attack on the radical empiricist's doctrine of what is "given," Wilfred Sellars (1963) claimed that it is an illusion to suppose that we can form thoughts about appearances that are independent of thoughts about objective reality. So suppose, for example, that the epistemological solipsist claims to know that something looks red to him, or that it appears as if something is red. That epistemological solipsist claims that knowledge that there actually is a physical object that is red is more tenuous, less secure, than knowledge of the subjective appearance presented by such an object. But Sellars wants to know precisely what it means to say that it looks as if something is red. Sometimes we use "seems"/"appears" language to indicate tentative belief—R. M. Chisholm (1957) called this the epistemic use of "appears." But in its epistemic sense, the judgment that it appears as if X is red is just the tentative judgment that X is red—it is not a truth about an appearance to which one might appeal as evidence for the claim that there exists before one a red object.
There is another use of "appears," however—the comparative use. But it will not be of any use to the philosopher intent on restricting a knowledge claim to subjective experience. In the comparative sense, to judge that it appears to me as if X is red is just to judge that I am having the kind of experience that is usually caused by red things under normal conditions. It takes but a moment's reflection to realize that this thought about how things appear is not a thought confined to subjective reality at all. To know that it looks as if something is red, I would have to know something about objective reality—I would have to know how red things look under normal conditions—something that presupposes that I have had epistemic access to how things have been, not just how things appear.
If the only way that we could conceptualize experience was comparatively in the above sense, then it would be folly to suggest that our knowledge of reality begins with knowledge of subjective appearance. But, of course, it is not difficult to see how the epistemological solipsist should respond to the above criticism. The very characterization of the comparative use of "appears" seems to make reference to a "way" that red things look and the radical empiricist/epistemological solipsist thinks that we have no difficulty conceptualizing that way in terms of its intrinsic character. However the word "appears" is normally used, the epistemological solipsist can borrow that term to describe what Chisholm called the noncomparative intrinsic character of the experience (1957).
There are countless other attacks on the radical foundationalist's idea that all empirical knowledge rests on a foundation of knowledge about the character of subjective experience. Some, for example, argue that we must reject such a view because it will ultimately lead to a radical skepticism—perhaps even the metaphysical solipsism discussed earlier. The charge is that the foundations countenanced by such a view coupled with available epistemic principles simply will not allow us to get back the knowledge that we commonsensically take ourselves to have. To determine whether epistemological solipsism does lead to skepticism would take us too far afield, but one might wonder whether a commitment to the falsity of skepticism should rule philosophical thought.
Still others complain that the epistemological solipsist radically overintellectualizes the nature of our thought about external reality. Not only do we not always infer objective reality from subjective experience, we rarely pay attention to how things appear. As anyone who has tried to paint soon realizes, it takes a certain amount of learning and sophistication to see the world as it appears instead of as we take it to be objectively. But it is not clear what relevance this observation has for the epistemological solipsist's central thesis. To be sure, if the solipsist makes a claim about what we actually do know, the above observations might cast doubt on that claim by casting doubt on the question of whether we typically form the required thoughts. But the careful epistemological solipsist might make a claim only about the possibility of knowledge. That epistemological solipsist might argue that whatever we think we know or justifiably believe, the only truths that can be known, at least directly and without inference, are truths about the character of subjective experience.
Just as there is an internalism/externalism controversy concerning the nature of mental states, so also there is an internalism/externalism controversy in epistemology. Many epistemological externalists argue that whether or not a belief is justified depends critically on the causal history of the belief—the way in which the belief was produced. Alvin Goldman (1979) advances one version of such a view—reliabilism. The reliabilist is a kind of foundationalist but argues that foundationally justified beliefs are just beliefs produced by reliable belief-producing processes that take as their input something other than beliefs. A belief-producing process of this sort is reliable when it does or would produce mostly true beliefs. Reflection on the reliabilist's criterion for non-inferential justification reveals that there can be no a priori restrictions on which beliefs might turn out to be non-inferentially justified. Against the traditional foundationalists, the reliabilist will argue that non-inferential justification has nothing to do with infallibility. A belief can be non-inferentially justified if it is just barely more likely to be true than false. Beliefs about the past, the physical world, and other minds all might be non-inferentially justified according to the reliabilist. Whether they are or not depends on empirical facts about the way in which such beliefs are caused.
It is certainly true that arguments for epistemological solipsism are challenged by contemporary versions of epistemological externalism. It is hardly the case, however, that philosophers agree on the success of externalist analyses of epistemic concepts. The epistemological solipsists have an array of weapons ready to deploy against the externalist. But underlying their criticism is often the common theme that the externalist's analysis of epistemic concepts has stripped them of their philosophical interest. The epistemological solipsist is likely to be convinced that satisfying the externalist's epistemic concepts does nothing to provide assurance of the sort the philosopher seeks. I may have a reliably produced belief. I may be evolutionarily programmed to believe reliably various truths about the world around me. But unless I have some reason to believe that the way in which my beliefs are formed is reliable, the mere fact of reliability does nothing to give me the kind of assurance I was looking for when I was interested in having justified beliefs.
Another quite different sort of doctrine that might be associated with the idea of the self standing alone is the ethical theory or theory of rational behavior known as egoism. A crude version of the theory is that rational people have only one goal or end in acting—their own happiness or well-being. Egoists can certainly take into account the well-being of others but only insofar as they have some reason to believe that the well-being of others impacts their own well-being.
Like other versions of solipsism, egoism has been accused by some of internal incoherence. As a theory, one argument goes, egoism must enjoin everyone to achieve his or her own well-being. But we can easily imagine a case in which my doing X maximizes my well-being, while R 's preventing me from doing X will maximize R 's well-being. I cannot coherently recommend, exhort, or want R to prevent me from doing what I want to do.
The above criticism presupposes that a principle of morality or rationality must be universalizable in certain respects. More specifically, it presupposes that if someone accepts the principle that everyone ought to seek only his or her own well-being, that commits that person to recommending such behavior to others, or acquiescing in such behavior on the part of others, or wanting others to behave in such ways. Such presuppositions are not uncontroversial even in the domain of morality, but are arguably downright implausible if the "ought" judgment in question is intended to assert only the rationality of egoistic behavior. There seems nothing at all inconsistent in my believing that it would be rational for all people to act egoistically while encouraging them not to so act and doing what I can to prevent them from acting that way. I know all too well what people ought to do to beat me in a game of tennis, but I never advise them concerning how to do it; I never want them to do it, and I do whatever I can to thwart them from behaving as they ought to.
As an ethical theory, the plausibility of egoism might in the end depend on metaphysical issues concerning the nature of ethical properties. G. E. Moore (1912) famously argued that if my happiness is objectively good, it is so in virtue of the property of being happy that I exemplify. But if objective goodness "supervenes" in this sense on the property of being happy, then it supervenes on that property no matter whose happiness we are talking about. An ethical egoist cannot recognize the goodness of his or her own happiness without recognizing the value inherent in another person's being happy. But most egoists are not objectivists about value. On one view, diametrically opposed to ethical objectivism, something has intrinsic value for a person S only insofar as S subjectively values that thing for its own sake. And it is just a brute fact about most human beings, the egoist claims, that people care more about their own happiness than they do about the happiness of others.
That alleged empirical truth, however, is not uncontroversial. It might not be all that difficult for most parents, for example, to conclude that they value intrinsically the happiness of their children—perhaps even more than they value their own happiness. If they do, and if subjective valuing confers intrinsic value on that which is valued, then the egoist's view that rational people concern themselves only with their own well-being is implausible. It is worth noting, however, that the view according to which a thing's intrinsic value for a person is determined by that person's valuing it is itself a kind of solipsistic view. It is not egoism, because we might find ourselves valuing intrinsically the well-being of others, but it is still a view that makes the individual person the creator of the goals or ends that partially define for that person how life ought to be lived.
See also Augustine, St.; Ayer, Alfred Jules; Bradley, Francis Herbert; Bridgman, Percy William; Broad, Charlie Dunbar; Carnap, Rudolf; Descartes, René; Driesch, Hans Adolf Eduard; Egoism and Altruism; Epistemology; Fichte, Johann Gottlieb; Hamilton, William; Hume, David; Kant, Immanuel; Lewis, Clarence Irving; Locke, John; Mach, Ernst; Malcolm, Norman; Mill, John Stuart; Moore, George Edward; Other Minds; Pastore, Valentino Annibale; Private Language Problem; Royce, Josiah; Russell, Bertrand Arthur William; Santayana, George; Schiller, Ferdinand Canning Scott; Schuppe, Ernst Julius Wilhelm; Stace, Walter Terence; Stebbing, Lizzie Susan; Wittgenstein, Ludwig Josef Johann.
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Wright, Crispin, Barry C. Smith, and Cynthia Macdonald, eds. Knowing Our Own Minds. Oxford: Clarendon Press, 1998.
Richard A. Fumerton (2005)
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