The Scope of Knowledge
I must admit that I have never seen three people or objects that disconfirm the example regarding tallness, but imagine the following possible world.
Imagine a world with only three rods and one observer (you). When rod A is visually compared to B, A appears to be taller; when B is visually paired with C, B appears taller; but when C is compared to A, C appears taller.
When you attempt to place all three in the same visual field, you cannot take them all in by sight. One always disappears out of the visual field. This world is possible: in fact, it is not so unlike the world of quantum mechanics, with its indeterminacy and measurement problem. And the world described seems not to be a world in which the supposed necessary truth about tallness holds true. Hence it is not a necessary truth (unless we simply stipulate a definition of ‘tallness’ in which only a transitive relation will bear that term).
Even if we are not so sceptical of necessary truths about reality, the relation between appearing and being a real property of objects seems not to be necessary. If we are to answer sceptical challenges to our beliefs about that relation, we therefore require an inductive argument, broadly construed. We have looked at the implications for the sceptical argument of the counterfactual analysis of knowledge. It is time to use the analysis we endorsed, that knowledge is belief best explained (in significant part) by truth. Here the question becomes whether the ways we are appeared to in experience are better explained by objects and their properties as we take them to be than by sceptical hypotheses such as the programming of brains in vats. It will be objected to this approach right off that we cannot construe our beliefs about physical objects and their properties as a theory that best explains the patterns within appearances because we cannot even formulate such patterns without appeal to physical space and objects. We learn the language of appearing only after learning to hedge our claims about real properties, which we immediately and naturally ascribe without inference. More important, it is claimed, we know what experiences we have had and will have only by reference to locations in physical space among physical objects. Thus, to even formulate the supposed data we must presuppose the ‘theory’ that is supposed to explain the data, which ‘theory’ therefore does not require this explanatory justification.
The former point about learning, however, is again irrelevant to the question of justification, or demonstrating knowledge. Even if we must conceive of physical properties first, that does not show that there are such properties or that they are as we take them to be. Our evidence for our beliefs about them still consists in the ways they appear, although we don’t initially conceive it that way. The latter point about formulating the data is first of all debatable. While we cannot translate statements about physical properties into statements about appearings, it is not so clear that we could not learn to weaken all claims about objects to claims about appearances, although doing so would be very cumbersome and awkward. Instead of talking of seeing an unsupported object fall, we could talk of a visual experience as of an unsupported object followed by a visual experience as of an object falling. But even if such reductions would not be universally possible for a skillful language user, this still only indicates a conceptual necessity, the way we must express ourselves. It still does not imply that the ways we conceive objects are the ways they are independent of our conceptions, and it still does not preclude the attempt to show that real properties as we conceive them best explain the ways they appear to us.
That physical objects and their properties do provide the best explanation we have for the ways we are appeared to seems easily established. Appeal to real physical objects first of all explains more deeply than do explanations for particular appearances in terms of regularities within experience itself. The former can explain the regularities themselves that are otherwise taken to be ultimate. And explanations in terms of physical objects and their properties are also superior to sceptical explanatory hypotheses such as programmes for brains in vats or deceptive Cartesian demons. The agreement of the different sense modalities on the qualities and dimensions of objects, for example, strongly suggests a realist explanation, and of course we also use the realist model successfully to predict future experiences. The physical realist picture explains both how physical objects interact with each other and, at least in part, how they cause experiences (by reflecting light, emitting sound waves, and so on). No such predictions or explanations are forthcoming from the brain in vat or evil demon hypotheses, which are therefore ad hoc and useless additions. The only way they have any explanatory power is by being parasitic on physical object explanations: the demon or programmer of brains must make it seem as if we are surrounded by interacting physical objects (Vogel 1990). Not only does this add nothing to the explanations available without these additions; it also raises natural but unanswerable questions, such as why the programmers or demon would deceive us in this way.
If the commonsense and scientific explanations for our experience are superior to the sceptical hypotheses, does this show that we know that we are not brains in vats? When a hypothesis is put forth only to explain certain data, and when a superior explanation is later offered, we have some reason to disbelieve the former hypothesis and the entities it posits solely for explanatory purposes. When the demon theory of disease was replaced by the germ theory, rational people ceased to believe in disease-causing demons. In that case, however, there was additional evidence against the existence of demons – the fact that no one has ever seen one (except perhaps in a highly irrational frame of mind). The best explanation for the latter fact is that there are none. In the brain in the vats case, we would not see ourselves as such if we were, and so we lack that additional evidence. Unlike the usual case of knowledge of negative existential propositions (propositions that certain things do not exist), we have no evidence against the existence of the brains (while also lacking evidence for their existence). It is this, we can claim, that explains the plausibility of the sceptical argument, while leaving it open to us to defend our knowledge claims against that argument via inference to the best explanation. Two problems remain for this inference as an answer to the sceptic. The more tractable one is that an inference to the best available explanation at a given time is not necessarily an inference to the best explanation tout court, so that even if we can accept the latter as producing the true explanation, we cannot so easily accept the former as doing so. We need additional reason to think that the available explanations exhaust the field of plausible ones. Here that demand seems easily met, however. There does not seem to be the remotest possibility of an explanation for our experience being developed that rivals that which appeals to physical objects and the scientific theories of physical reality.