Case Study: The External World Hypothesis

Jonathan Vogel has argued that the strategy of inference to the best explanation can show that we have reason to believe that we are not brains in vats and that we have experience of a physical world (Vogel 1990).⁹ Vogel presents the issue in terms of how to make a rational selec-tion between two rival hypotheses. According to the external world hypothesis we have experience of a physical world and at least many of our common sense beliefs about what the physical world is like are true. According to the brain in a vat hypothesis, there exists only your brain and a computer feeding it data. All that you experience are the data, but the data deceive you by giving every appearance that you are experiencing a world of physical objects. For every physical object that the external world hypothesis posits, and for every observable property of such an object, the brain in a vat hypothesis posits a corresponding object with a corresponding property. The corresponding object will be a digital object: some information stored in the computer memory. For example, suppose that according to the external world hypothesis there is a frog in the garden pond. Then, according to the brain in a vat hypothesis, the computer programme will hold a file about a frog in the pond. Similarly, for every physical property the external world hypothesis attributes to the

9 See also Weintraub (1997, 116–20) and Feldman (2003, 148–52).

frog, the brain in a vat hypothesis will attribute a corresponding, though different, property to the file about the frog.

In this way, then, the two hypotheses cannot be distinguished by our experiences alone. But Vogel thinks that the hypotheses differ in their degree of complexity. He seeks to bring this out by focusing on the issue of location properties — the properties of being located in a certain place at a certain time. The external world hypothesis attributes locations in space and time to physical objects. Vogel, along with many other phi-losophers, think that physical objects are governed by the principle that non-identical physical objects are not located in the same place at the same time. The frog is in the pond. According to this principle, another frog cannot occupy the same place at the same time as the other frog.

According to Vogel, the principle that physical objects of the same kind are not located at the same place at the same time is a necessary truth.

“We do not need any empirical law or regularity to explain [the prin-ciple]; it is a necessary truth pertaining to the nature of physical objects that there cannot be two such objects in the same place at the same time”

(Vogel 1990, 664). (Vogel’s principle is controversial. Three-dimensional objects such as a statue and a lump of bronze can be located in the same place at the same time. Yet, it can be argued, the statue and the lump of bronze are non-identical because the bronze continues to exist even if the statue is melted down and the bronze is formed into a ball. Vogel would deny these objects belong to the same kind, but is that ad hoc?)

Now if the brain in a vat hypothesis is to match the external world hypothesis in explanatory power, the former has to have a corresponding principle. It needs to have a principle that says that non-identical digi-tal objects are not represented as having the same spatial location at the same time. (The computer programme might represent where an object is located in space and time by assigning a string of co-ordinates in its file. Call this property the object’s “pseudo location.”) This principle, how-ever, is not a necessary truth. Nothing in the nature of a digital object, or of a computer programme, excludes such a programme taking non-identical digital objects as having the same pseudo location. Therefore, Vogel says, the brain in a vat hypothesis has to take the principle to be “an extra empirical regularity, to which no regularity in the [external world hypothesis] corresponds” (Vogel 1990, 665). Consequently, by adding such an empirical regularity, the sceptical hypothesis is less simple than the external world hypothesis; and if it omits it, the sceptical hypothesis is less explanatory than its rival. Inference to the best explanation says that the hypothesis that is more explanatory than a rival is more likely to be the correct hypothesis. We are then licensed to infer that the external world hypothesis is the more likely hypothesis.

Ernest Sosa thinks that appeals to explanatory considerations to meet external world scepticism are flawed. Such appeals claim that we can know that there are external objects because it provides the best explana-tion of our sensory experiences. Consequently, it is further claimed that we know that we are not dreaming. Sosa asks “How plausible can it be that one could discover that one is not dreaming as a by-product of any such inference?” He offers the following analogy:

Compare a case where one knows about one’s current speed only by reading one’s speedometer. And suppose this knowl-edge to depend entirely on an explanatory inference from the speedometer reading to the actual speed. Could one thereby discover, and come to know, that one’s speedometer was prop-erly connected so as to be sensitive to one’s speed? If one asked oneself whether the speedometer was properly installed and operative, one could hardly settle the matter exclusively by inferring one’s actual speed from the speedometer reading, as an explanatory inference, so as to draw the further inference that the instrument must indeed be properly installed and operative. (Sosa 2007, 59)

In this passage Sosa asks whether we could get to know whether the speedometer is reliable by having inferred that the speedometer’s read-ing is explained by our current speed. He claims that we could not, but we need to tease out what his reason for claiming this is. He says that we could not know that the speedometer is reliable exclusively by having inferred that the speedometer’s reading is explained by our current speed.

This, however, is precisely what is at issue. Sosa thinks that an explana-tory inference cannot provide knowledge in a case in which we have to rely exclusively on that inference. Sosa’s opponents believe that explana-tory inferences are a source of knowledge, and that, at least in some cases, those inferences provide us with knowledge (or at least justified belief) that we would otherwise lack. For instance, Vogel thinks that explanatory inference is a source of our knowledge of the external world, and that we have to rely exclusively on that inference to have that knowledge. That is, he thinks that our only source of the external world is an explanatory inference from our sensory experiences. So Sosa assumes the very point at issue in the debate with Vogel.

In assuming that an explanatory inference cannot provide knowledge in a case in which we have to rely exclusively on that inference, Sosa is apparently assuming that an explanatory inference provides knowledge that p only if there is some independent means of discovering whether p is

true. We may make an explanatory inference from the speedometer read-ing to our current speed. Yet unless there is some way of tellread-ing what our current speed is independently of making this inference, we cannot know what our speed is, and we cannot know whether the speedometer is reli-able. Similarly, we may make an explanatory inference from our sensory experience to the existence of an external world. Yet unless there is some way of telling whether there is an external world independently of making this inference, we cannot know whether there is an external world.

There is reason to reject this assumption that there has to be an independent means of testing the explanatory inference. Philosophers of science have recognized that some explanations are self-evidencing (Hempel 1965, 370–74). In a self-evidencing explanation, a theory T explains a phenomenon P although P provides the only evidence for T.

For example, the Doppler law links the recession of an object with the red-shift (a distinctive spectrum that is shifted towards the red). That law, in conjunction with the fact that a given galaxy is receding from us at a certain velocity, explains the fact that the galaxy exhibits the red-shift.

Yet the only evidence that the galaxy is receding at that velocity is the fact that the galaxy is exhibiting the red-shift. It is standardly thought by philosophers of science that such a pattern of reasoning is neither worthless nor viciously circular. It is not viciously circular because one relation (the explanation relation) holds between the explanans and the explanandum (in that order), while a distinct relation (the confirmation relation) holds between the explanandum and the explanans (in that order). It is not worthless because the only way in which we can have knowledge of unobservable scientific entities is via an explanatory infer-ence from observed entities, and (scientific realists will claim) we do have some such knowledge.

It is open to Vogel to claim that the explanatory inference from our sensory experience to the existence of an external world is a self-evidenc-ing explanation, and that so too is the inference in Sosa’s speedometer example. Those inferences are none the worse for that if, as seems to be the case, self-evidencing explanations are widely accepted in current sci-entific practice. Of course, Sosa might wish to take issue with this trend in science, but then he would need to say what is in general wrong with self-evidencing explanations.

It is also open to Vogel to claim that any persuasive power in Sosa’s example of the speedometer does not lie in any supposed vicious circu-larity in the speed of the car explaining the speedometer’s reading and the speedometer’s reading being the only evidence for the speed of the car. Instead, given how minimally the example is described, that seems no reason to claim that the best explanation of the speedometer’s reading

is that it is reliably tracking the speed of the car. The example does not tell us anything about the workings of the speedometer — whether it is a standard model, whether it has been tampered with, and so on. Given this paucity of information, we may be disinclined to think that the reli-ability hypothesis is a much better hypothesis than its nearest rivals. In the case of the problem of the external world, however, we have seen that Vogel thinks that the external world hypothesis is markedly simpler than, and in at least that respect better than, the sceptical hypothesis.

(Curiously, Vogel had already devised an example similar to Sosa’s and directed it against reliabilism in epistemology [Vogel 2000, 613]. Sosa’s example might then serve as an ad hominem.)

What is crucial to Vogel’s argument is the claim that the sceptical hypothesis is less simple than the external world hypothesis. Vogel’s reason for making this claim is that the sceptical hypothesis posits an empirical regularity although the external world hypothesis posits no corresponding hypothesis. Whether this shows that the sceptical hypothesis is less simple is debatable for two reasons. First, according to Vogel, the external world hypothesis posits a necessary truth — namely, that non-identical physical objects of the same kind do not occupy the same space-time location — although the sceptical hypothesis posits no corresponding necessary truth. So the situation seems to be that the sceptical hypothesis posits an empirical regularity to which the exter-nal world hypothesis posits no corresponding empirical regularity, and that the external world hypothesis posits a necessary truth to which the sceptical hypothesis posits no corresponding necessary truth. But then it no longer seems that the sceptical hypothesis is a less simple hypoth-esis than the external world hypothhypoth-esis; they seem to be equally simple (equally complicated) hypotheses. Second, we should re-consider the assumption that the external world hypothesis does not posit an empir-ical regularity corresponding to the sceptempir-ical hypothesis’s empirempir-ical regularity. According to Vogel, it is a necessary truth that non-identical physical objects of the same kind do not occupy the same space-time location. But that (alleged) necessary truth entails the empirical gen-eralization that non-identical physical objects of the same kind do not occupy the same space-time location. If it is a necessary truth about the nature of physical objects that physical objects of the same kind do not occupy the same location in space and time, then it is an observationally discoverable general fact that physical objects of the same kind do not occupy the same location in space and time. So it is not the case that the external world hypothesis lacks an empirical regularity corresponding to the sceptical hypothesis’s empirical regularity that non-identical digital objects do not have the same pseudo-location. Vogel’s case for claiming

that the external world hypothesis is more simple than the sceptical hypothesis seems to be mistaken.

Furthermore, since the external world hypothesis posits a necessary truth while the sceptical hypothesis does not posit a corresponding necessary truth, it seems that the external world hypothesis is less simple than the sceptical hypothesis. One version of Ockham’s Razor says that necessities are not to be multiplied beyond necessity. (Forrest [2001, 93]

calls this “Hume’s Razor.”) The external world hypothesis posits a neces-sity that the sceptical hypothesis does not. Still, even if Vogel were to concede this, he might reply that even what the external world hypothesis loses in simplicity, it gains in explanatory power. For the hypothesis can explain the empirical regularity that no non-identical physical objects of the same kind are in the same location in space and time. It is explained by the necessary truth that no non-identical physical objects of the same kind are in the same location in space and time. Since the sceptical hypothesis does not posit a corresponding necessary truth, it cannot explain in this way its corresponding empirical regularity. And none of the other empirical regularities that the hypothesis posits seem able to explain the empirical regularity in question. So the sceptical hypothesis cannot explain the regularity at all.

Which (if either) hypothesis should we accept? The hypothesis that explains more by being less simple, or the hypothesis that is more simple by explaining less? The strategy of inference to the best explanation needs to be supplemented not only by detailed accounts of each of the theor-etical virtues, but also by a detailed account of how to make a rational theory choice in cases such as the above.¹⁰

8. Conclusion

Philosophers who have thought that there was such a thing as philo-sophical explanation have tended simply to assume that there was such a thing. They have not thought that their assumption needed to be argued.

Even Kant, who billed himself as a critic of rationalist metaphysics and its explanatory pretensions, seemed to have thought that he was stating the proper way of giving philosophical explanations: we perceive the world as a world containing Fs (causes and effects, substances in space and time, …) because our minds structure our perceptions that way. For Kant, there is no question that philosophy is in the business of explana-tion. His chief difference with his rationalist opponents is that he takes

10 For further criticism of the use of this strategy to solve sceptical problems, see Wright (1985, 68–71), Fumerton (1992), and Vahid (2001, §2).

the locus of this explanation to lie in special powers of the mind. Kant did not think that these powers were to be discovered by empirical psychol-ogy. They were the special domain of synthetic a priori knowledge.

Serious challenges to the idea of philosophical explanation emerged only in the work of Hume, Mach and the logical positivists. The chal-lenges were obscured by more prominent ideas in their work — Hume and Mach’s stringent empiricism and logical positivism’s verificationist theory of meaning. But simply rebutting these views does not meet the challenge about explanation. Grant (against the logical positivists) that talk about unobservable things can be meaningful. Grant too (against Hume and Mach) that there can be epistemic reason to believe talk about such things. There remains, though, the challenge of how philosophy’s positing such things is supposed to explain anything. What kind of expla-nation is involved? How does it work? This was the point of departure for the present chapter.

In closing, three very recent approaches to philosophical explanation are worth noting.