Skepticism is the view that we have no knowledge at all, so that any talk of the nature of reality is pointless. Some ancient Greek philosophers advocated an extreme form of skepticism according to which neither sensation nor opinion could give us any grounds for separating truth from falsehood. An influential current form of skepticism is found in postmodernist philosophers and literary theorists who view the world as a text open to many kinds of interpretations, none of them demonstrably better than the others. In fields such as history, anthropology, and cultural studies, it has become fashionable to claim that reality is just a social construction, so that the idea of objective knowledge is only a myth.
Empiricism tries to avoid skeptical problems by restricting knowledge to what can be perceived by the senses. From early modern philosophers such as John Locke and David Hume to later thinkers such as Rudolf Carnap and Bas van Fraassen, the restriction of knowledge to sense experience has had strong appeal. I will show, however, that strict empiricism is incompatible both with the neuropsychology of perception and with the practice of science. Our brain processes are, fortunately, capable of reliably taking us well beyond what is presented to us by our senses.
Another approach to understanding knowledge of reality is idealism, which views reality as dependent on or even constituted by minds. This view is more compatible than is empiricism with the constructive nature of perception and inference, but grossly overestimates the contributions that minds make to the world. It leaps from the insight that there is no knowledge of things without construction of mental representations of them to the conclusion that entities are mental constructions. The philosopher Immanuel Kant thought that he had accomplished a kind of Copernican revolution by placing mind at the center of knowledge and reality. But idealism is actually attempting a kind of Ptolemaic counterrevolution, as implausible as reactionary attempts to return the earth to the center of the solar system or to deny human evolution.
There is an old baseball story about three umpires calling balls and strikes. One says, "I call them as I see them." The second says, "I call them as they are." The third insists, "They ain't nothing until I call them." These attitudes correspond to the philosophical positions of empiricism, realism, and idealism. For neuroscience to support realism about objects, I need to show that the structures and processes used by the brain enable it to represent things in the world as they are, at least approximately.
How do I know this reality exists?
First, I do not have to rely exclusively on a single sense. I see the color and shape of the loaf of bread, but I can cross-check the shape using my sense of touch, confirming that it feels the same way that it looks. I can also use hearing to investigate the bread by banging the loaf against a pot and hearing the ding. Further, the bread produces pleasurable stimulation of my senses of taste and smell. The brain has different sensory systems but can combine them to form unified perceptions. In contrast to hallucinations and dreams, which are hard to control, systematic experiments are possible: I can generate integrated and coherent sensations of the bread for example, by simultaneously looking at it, scratching it, and eating it. Because I can make the bread cause these experiences, and because there is no evidence to support alternative hypotheses (e.g., I am hallucinating or dreaming), it is reasonable to conclude that the bread exists. Its reality is the best explanation of my diverse experience of it.
Second, evidence for the reality of objects does not have to rely only on my own specific sensory experiences of them, as I can also often rely on the testimony of others. Any doubts I have about the bread’s causing my experiences can be reduced if I share it with other people, who will generally report similar experiences. You may not like this whole-grain bread as much as I do, but I would be very surprised if your reports of its color, shape, texture, smell, and taste turned out to he much different from mine. We can make a party of it and have a bread tasting in which we all compare our sensory experiences. I predict that reports of the sight, feel, taste, smell, and sound of the bread will be remarkably convergent. The best explanation of this convergence across the sensory experiences of multiple people is that there really is a loaf of bread that is causing all of our brains to generate similar experiences. The reports of similar experiences by me and other people all result from a combination of physical mechanisms by which the bread affects our senses and neural mechanisms by which our brains interpret sensory inputs.
But should we rely on the testimony of other people as part of our inference to the best explanation of sensory reports? After all, they might be lying or joking, rather than actually reporting their experience of the bread. Once again, our assessment of the truth of what people say to us is a matter of inference to the best explanation. You are justified in believing that someone is telling the truth if that is the best available explanation of all the available evidence. People are usually motivated to describe things as they think they are, so you are justified in taking what they say as relevant evidence, as long as there isn’t evidence supporting alternative hypothesis such as deception or hallucination. Testimony justified by inference to the best explanation allows me to reasonably believe many things observed by others. I have never been to Mount Everest myself but do not doubt its existence, because the observational reports of many others are better explained by the hypothesis that the mountain exists than by alternative hypotheses such as mass deception.
But how do we know that the experiences reported by other people are at all the same as ours? Maybe when you say you are experiencing brown, chewy bread, you are really having the same experience I have when I experience white, soggy bread. There are two reasons for doubting that there is sufficient variability in experience to undermine the usefulness of testimony First, the general pattern of experiences that people usually report has a great deal of overall coherence with my pattern of experience, which makes it implausible that we differ in just one kind of experience such as brown or chewy. Second, there is much evidence from anatomy and brainscanning experiments to suggest that people’s brains are very similar for sensory processing. Hence there is good reason to take the testimonial reports of other people at face value, in the absence of evidence that they are lying or demented.
In addition to multisensory coherence and the testimony of other people, there is a third reason for inferring that our perceptions of objects are approximately true: we can often corroborate them with measurements taken by instruments. People don’t usually subject a loaf of bread to instrumental inspection, but a physicist could use calipers to measure its height and width, a spectrometer to measure the color reflectance of the loaf, an artificial odor detector to measure molecules near the loaf, and so on. Such measurements carried out by people or possibly even by robots provide further evidence best explained by the supposition that the loaf of bread exists. Similar arguments support inference to the existence of many other kinds of objects, from lions to mountains. Contrary to empiricism, scientific knowledge does not come just from our senses, but goes beyond them via a multitude of reliable instruments from telescopes and microscopes to Geiger counters (used to measure radiation) and particle colliders (used to detect the behavior of subatomic particles). The efficacy of scientific instruments is incompatible with idealism, because their measurements do not depend on mental activity, but it fits well with constructive realism.
You might think that even if pieces of bread are real, their properties (color, taste, smell, and texture) are not, because these are so heavily dependent on our minds. Many philosophers have thought that nothing in the external world corresponds to people’s experiences of colors, eliminating them as real. Their arguments rely on the fact that there is no simple mapping between the space of colors that people experience and the properties of objects that affect how they reflect light of different wavelengths. Paul Churchland has found, however, a way of construing the physical properties of objects that reveals a correspondence between their reflectance efficiencies and people’s experiences of colors like red, green, and blue. He describes how the human visual system successfully tracks approximations of the reflectance profiles of objects at a low level of resolution, so that colors can be viewed as objectively real properties of objects even if color vision is highly context sensitive.
The correspondence between reflection properties and color experience makes sense given current theories of how the brain processes color information, from stimulation of cells in the retina that code for specific wavelengths of light to interpretations generated in the visual cortex. I like the conclusion that colors are real properties of objects, and it does seem to fit with the best available understanding of how the brain interacts with objects. But realism about objects could be true even if realism about colors is not, as long as we have good reason to believe that objects and at least some of their properties exist independently of mental representations of them.
I have tried to show in this section that the best explanation of the convergence of experiences from the multiple senses of many people and instruments is that there really are physical objects that cause these experiences. Moreover, the observable properties of these objects are much as we perceive them to be. Of course, they have other nonobservable properties, such as their atomic structure, that we can learn about only from scientific theorizing.
In sum, attention to how the brain functions in perception supports constructive realism over empiricism and idealism. The constructive nature of perception with both top-down and bottom-up processing shows the implausibility of a narrow empiricism that ties knowledge too closely to sensory input. On the other hand, the robustness of sensory inputs of different kinds counts by inference to the best explanation against the idealist view that the existence of objects is mind dependent. Our perceptual knowledge is both constructed and about real things. Such constructive realism is also the best approach to theoretical knowledge that uses concepts and hypotheses to go well beyond perception.