In this early chapter, Russell addresses one major issue—matter. He sets out to decide whether we can be sure that matter exists or if we must admit that matter is something imagined, as real as a dream might be said to be real. The criterion for our certainty is the independent existence of physical objects, for we have identified matter with physical objects in the preceding chapter. The object now is to establish what many philosophers suspect, that the table exists independent of our perception of it, that if we turn away from it the table is still there. Initially, Russell reminds us that while we are doubting the physical existence of an object, "we are not doubting the sense-data, which made us think there was a table," the immediate experiences of sensation.
If the table is real, then our confidence in our senses has been well-placed, and we might be said to have reasonably inferred reality from its appearance. If we find, with Russell, that the table is not real, then the "whole outer world is a dream." It is vital to grasp the difference between these two hypotheses. One affirms our common-sense view of reality, and the other holds that "we alone exist" and nothing we experience is real in our ordinary sense. Russell will contend that it cannot be proved that we are not dreaming "alone in a desert," but also argues that there is no reason for supposing that this is the case.
Here, Russell refers to Descartes' Meditations. Descartes believed in nothing that was not clearly and distinctly true. He imagined the possibility of a disordered, deceptive reality. Descartes considered the deceitful demon possible because he could not prove that it wasn't the case. However, Descartes found that it could not be the case that he himself did not exist; it was impossible because if he did not exist, then he could not be deceived by a demon. Since he doubted, he necessarily existed. Russell highlights Descartes' service to philosophy as that of illustrating that "subjective things are the most certain."
A formal statement of the problem goes: "Granted that we are certain of our own sense-data, have we any reason for regarding them as signs of the existence of something else, which we can call the physical object." The first reason that Russell examines involves the idea of public experience versus private experience. If a group of people is sitting together at a dinner party around a table, then it is reasonable to assume that they see the same forks and knives, the same tablecloth, the same glasses. Since the sense-data is private to each person, "what is immediately present to the sight of one is not immediately present to the sight of another," and it is reasonable to infer that "they all see things from slightly different points of view, and therefore see them slightly differently." Common experience suggests that we believe in such "public neutral objects," as Russell calls them. And if these objects are to exist, objects which more than one person can know, then it seems as though there must be something that transcends the private experience of sense-data. Further examining this reason for believing in the independent existence of physical objects, Russell next inquires as to why we should believe in public neutral objects.
It is true that though people's experiences may vary a little, they can be remarkably similar. It is also true that the variations between their descriptions may vary according to scientific principles having to do with perspective and reflection. However, at this moment Russell retreats from the height of this inquiry to point out that to the extent that we have admitted the experiences of other people, we have made a mistake. Supposing that other people exist begs the question at stake, since the existence of other people is predicated on the assumption that physical objects exist independently. In this stage of the argument, other people are only represented by sense-data. Russell's conclusion from this example is that we must make no appeal to sense- data outside our own private experience.
Here Russell acknowledges that, strictly speaking, we could never truly know that the whole outer world is not a dream. It is always a logical possibility that we are deceived about the true nature of reality and that it is hidden from us. It is possible because "no logical absurdity results from the hypothesis that the world consists of myself and my thoughts and feelings and sensations." However, Russell's argument is that though there may be no way to disprove this "uncomfortable" possibility, there is no reason in support of it either. What is simpler and more plausible is the hypothesis that independent physical objects exist "whose action on us causes our sensations." The advantage of this hypothesis is in its simplicity.
Russell analyzes an example of a moving and hungry cat in order to show the importance that simplicity plays in our reasoning. If a cat appears in one place in a room, then in the next moment it appears in another place, it is "natural," Russell says, to believe that the cat has moved. Yet, on the view of private experience that solely endorses sense-data, the cat could not have been in any other places besides where one sees him. He simply does not exist when one doesn't see him. Now, another possibility is that the cat does exist when one sees him and when one does not see him. He persists, and it is easy to infer that he becomes hungry between one meal and the next. Yet, Russell continues, if he does not exist when he is not observed, then it is harder to imagine him becoming hungry when he does not exist. Further still, if he does not exist independently and only exists as a piece of sense-data, then the idea of his being hungry is unintelligible anyway. On this view, one can only experience one's own hunger as a piece of sense-data. Russell writes that the "expression of hunger becomes utterly inexplicable when (the cat is) regarded as mere movements and changes of patches of color," as sense-data.
Russell then extends his cat example to other people. When watching someone speak, we perceive sense-data like moving lips and uttered sounds, and we naturally believe that another person is expressing his thoughts, much as we would if we acted in a similar way. We draw an unconscious analogy between how we think of our own behavior and how we perceive the behavior of others. That is, it is difficult to imagine that the person does not exist independently.
Here Russell discards the hypothesis that our notion of reality is just a case of dreaming. We know that in dreams one perceives the presence of other people and realizes this later to be a mistake. Russell claims that dreams are suggested by "waking life" and can be "accounted for on scientific principles if we assume there is a physical world," that is, we assume that our dreams are patterned on a reality where we probably do perceive real people. Though he admits that the dreaming case will always pose a "slight doubt," he reasons by that independent reality is the simpler hypothesis. It is the "natural view" urged by "every principle of simplicity," that we are experiencing real, physical objects that exist outside ourselves and do not depend on our perception for their existence.
Russell concludes his assertion of simplicity with a look at why the view seems natural in the first place. We do not first adopt the view that there are other people beside ourselves because someone has successfully argued the case before us. The independent existence of reality is a natural belief because "we find this belief ready in ourselves as soon as we start to reflect." Russell calls it an instinctive belief. He points out that we have only doubted the external world because it failed to be identical with our sense-data. Yet, physical objects still seem to correspond to our sense-data. This instinctive belief simplifies our thinking about our experiences, rather than complicating it, and thus there seems to be no reason not to accept the common sense hypothesis over the dreaming hypothesis.
He ends the chapter acknowledging that the argument for simplicity is perhaps weaker than we would have it but typical of most philosophical argument. Briefly, he conceives a theory about the hierarchical character of knowledge. He writes, "all knowledge, we find, must be built up upon our instinctive beliefs, and if these are rejected, nothing is left. But among our instinctive beliefs some are much stronger than others, while many have, by habit and association, become entangled with other beliefs, not really instinctive, but falsely supposed to be part of what is believed instinctively."
In the course of his discussion Russell offers a very brief but sophisticated reading of Cartesian certainty. He poses the classic problem of understanding personal identity examining the "I" of Descartes' famous "I think, therefore I am." His speculation that the "real Self is as hard to arrive at as the real table, and does not seem to have that absolute, convincing certainty that belongs to immediate experiences (sense-data)," posits a fundamental doubt that we are the same person today as we were the day before. The passage is an example of The Problems of Philosophy in its capacity as introduction; however the problem of personal identity is a side issue.
The problem of the "other" mind looms quietly in the background of this chapter's considerations. The pursuit of the foundation of the independent existence of other objects includes the independent existence of other people. We are not only dreaming but are "dreaming alone." If we are unsure about the status of reality, then we cannot be sure of other people's bodies or their minds.
Though, as Russell points out, there is no way to prove that reality is not just a dream (it is logically possible that you are dreaming as you "read" these words on the computer screen), there is no reason why this should be the case. He writes, "we cannot have reason to reject a belief except on the ground of some other belief." The hypothesis that we are dreaming our experiences is no more plausible than our common-sense hypothesis; we have no further evidence that it is the case. Since there is no ground for other belief, the most natural possibility among possibilities to accept is our common-sense view, belief in an independent external world.
The end of this chapter culminates in a soaring expression of Neo-Platonic ideology. Just as Socrates and Plato believed that a body of truth was intrinsically coherent and that in seeking consistency we reject false beliefs, Russell believes that philosophy "should show us the hierarchy of our instinctive beliefs, beginning with those we hold most strongly, and presenting each as much isolated and as free from irrelevant additions as possible our instinctive beliefs do not clash, but form a harmonious system. There can never be any reason for rejecting one instinctive belief except that it clashes with the others; thus, if they are found to harmonize, the whole system becomes worthy of acceptance." Russell advocates a "systematic organization" of philosophy and of knowledge. He admits that the possibility of error remains for us, but its "likelihood is diminished" by scrutinizing each part of the whole theory.
Russell's formulation that "the whole outer world is nothing but a dream" has been reformulated by subsequent philosophers, including Hilary Putnam, who posited the famous case of a brain in a vat that is "sensitized" by the medium in the vat, but experiences life as we experience it. The idea of a false reality, which is false in the sense that our experiences do not resemble reality as it really is, finds expression in such popular examples as the 1999 motion picture The Matrix.