The guarantor

Commenting on geocentrism, van Inwagen says:

Why did the medievals believe this? Well, because that's how things felt (the earth beneath our feet feels as if it were not moving) and that's how things looked. Today we know that the astronomical system accepted by the medievals–and by the ancient Greeks from whom the medievals inherited it–is wrong. We know that the medievals, and the Greeks before them, were derived by appearances. We know that while the solid earth beneath our feet may seem to be stationary, it in fact rotates on its axis once every twenty-four hours. (Of course, we also know that it resolves around the sun, but let's consider only its rotation on its axis). Now suppose you were standing on a merry-go-round and were wearing a blindfold. Would you be able to tell whether the merry-go-round was turning or stationary? Certainly you would: passengers on a turning merry-go-round feel vibration and the rush of moving air and, in certain circumstances, a hard-to-describe sort of "pulling." (This last will be very evident to someone who tries to walk toward or away from the center of a turning merry-go-round. These effects provide the "cues," other than visual cues, that we employ in everyday life to tell whether we are undergoing some sort of circular motion. The medievals and the ancient Greeks assumed that because they did not experience these cues when they were standing or walking about on the surface of the earth, the earth was therefore not rotating. Today we can see their mistake. "Passengers" on the earth do not experience vibration because the earth is spinning freely in what is essentially a vacuum. When they move about on the surface of the earth, they do not experience the "pulling" referred to above because this effect, though present, is not sufficiently great to be detectable by the unaided senses. And they do not experience a rush of moving air because the air is carried along with the moving surface of the earth and is thus moving relative to them.  

This example shows that it is sometimes possible to "get behind" the appearances the world presents us with and to discover how things really are: we have discovered that the earth is really rotating, despite the fact that it is apparently stationary….We talk about reality only when there is a misleading appearance to be "got behind" or "seen through". P. van Inwagen, Metaphysics (Westview Press, 4th ed, 2015), 2-3.

1. This raises a number of philosophically and theologically significant issues. To begin with, there are different kinds of realism–or should I say, realisms?

For instance, I might be a metaphysical realist about material reality. I believe there's a material reality that's causing my sensations. Yet I might be an epistemological antirealist if I'm skeptical about what can be known regarding the material reality that's causing my sensations. 

Likewise, I might be a metaphysical realist about immaterial reality. I might believe in mental entities, viz. God, angels, souls, abstract objects. (I classify these as mental entities.) And I might be an epistemological realist about what can be know regarding immaterial reality. That's because, if immaterial reality is knowable, the source of knowledge is different than in the case of material reality. In the case of material reality, the source of knowledge is sensory perception, whereas, in the case of immaterial reality, the source of knowledge is reason and revelation. Inference, intuition, and divine disclosure. 

2. Up to a point, I agree with Inwagen. Metaphysically speaking, it isn't appearances all the way down. Something objective is producing the appearances.

In addition, I agree with Inwagen that there can be cues which indicate that appearances don't tell the whole story. That, however, is different from the claim that we can get behind appearances to discover reality. Epistemologically speaking, it may be appearances all the way down.

For instance, a colored object has a different appearance if I'm color blind. Likewise, creatures have different kinds of color vision.

The same physical object (i.e. organism) will have a different appearance if seen by infrared vision. In that case we're seeing heat signatures.

And that's just on the surface. It will have a different appearance if seen through an MRI or electron microscope.

3. Another complication is that Inwagen is using sensory perception to correct sensory perception. That's unavoidable, but it raises the specter of circularity. What makes one set of sensory perceptions the benchmark for assessing another set of sensory perceptions? One justification might be that some sensory perceptions have more explanatory value. They point to an underlying cause or mechanism. 

4. There is, though, perhaps an even deeper issue. Consider an illustration. I can photograph a tree with a (digital) cellphone camera, then send that image to someone else. I don't know the technicalities, but I assume the image is encoded as electronic information, transmitted in that encoded form, then decoded at the other end. There's built-in software that retranslates the encoded image so that the recipient sees the same image as the sender.

Indeed, you could have two people standing side-by-side. They can directly compare the original image with the transmitted image. And the two images exactly match. 

We might say that's analogous to sensory perception, when our mental representation matches the sensible object. Or is it?

Suppose we approach this from the standpoint of naturalism. How is it possible for a mindless, nonpurposive process to create a coded transmission system in which the output matches the input? That's completely unlike my example, in which a camera is designed to produce as accurate visual reproduction. In which technology is designed to produce a matching image at the receiving end of the transmission. In which designers can compare the original with the output to ensure that the translation software decodes the information to yield a matching image. 

5. A naturalist might counter that if there's a mismatch between input and output, then organisms which depend on sensory perception won't survive. If, say, gazelles misperceive leopards, gazelles will become extinct. There are, however, problems with that explanation:

i) According to Darwinism (e.g. David Raup), 99.9% of species that ever existed have, in fact, become extinct. So that appeal seems to be self-defeating.

ii) Even if survival does depend on that correspondence, and even if survival provides evidence for the success of that correspondence, it doesn't follow that this is explicable on naturalistic grounds.

iii) Does survival depend on that correspondence? Consider an illustration. You produce piano music by depressing certain keys on the piano, simultaneously or in a particular sequence. But the music bears no resemblance to the keyboard. 

Rather, there's a causal correlation between depressing certain keys and producing certain sounds. By the same token, what if sensory perception operates on a similar principle? The mental representation might be very different than what produces the mental representation, but so long as these are systematically aligned in a cause/effect relation, an organism might be successfully responsive to its environment. 

6. Finally, who or what is the guarantor that sensory perception is reliable? Without God, there's no presumption that the input and output can even match up or at least correlate (4-5).

An atheist might say that cuts both ways: how can a theist rule out the Cartesian demon?

However, these are asymmetrical alternatives. On the one hand there's the indemonstrable possibility that God designed sensory perception to deceive us. On the other hand there's the demonstrable impossibility that sensory perception is reliable unless God designed it.