In Chapter 4 of Mind and Cosmos, Thomas Nagel returns to the theme of intelligibility which he introduced in Chapter 2. He raises a profound question about our reasoning capability its accuracy and its reliability. The question is this: is our reasoning capability a product of evolution or is it more like an invention or discovery about the world?
Perhaps the best way to explain Nagel’s point is to compare two categorically different human capabilities. One capability is physical for which vision can be used as an example. We can safely assume that vision has been shaped by natural selection. Over the course of millions of years, our ability to see the world around us accurately has been instrumental to our survival. We can even create a narrative for how it might be improved in coming centuries. People currently have differing abilities to see different colors. The ability to see color is determined by the sensitivity and number of color receptors in the retina. Most people have three different types of color receptors which we can call “blue,” “green” and “red.” Therefore, most people can see all colors that can be represented by a combination of blue, green and red.
A significant fraction of women can see a fourth color. They have a gene which produces a color receptor that is sensitive to one more color, so they can see blue, green, red-1 and red-2. These four colors can combine into many more total colors than can be seen by people with only three color receptors in the eye. Since four color vision is a sex-linked ability, it might be difficult to create an explanation as to how it might become necessary or available to all humans, but it is conceptually possible to envision such an outcome.
This same reasoning about vision can be applied to some mental events and dispositions. As a general rule, all humans are born with certain emotional and mental dispositions forged through long eons of evolution. For example, we generally have the ability to distinguish a friendly face from a threatening face. We will generally perceive the threatening face quicker than the friendly face. One unfriendly face in a crowd can generally be perceived more readily than the single friendly face. We come with evolutionary baggage that can be compensated for, but only if we take the time to apply reason to the experience and re-train our thinking.
Once reason is acknowledged as a factor, then it, too, becomes subject to inquiries about its relationship to the evolutionary process. Reason can be named the non-physical human capability or the mental capability. If we apply the same narratives as we used above for the physical capabilities then we are left with some important questions. Can evolution improve upon our basic reasoning ability? Might our reasoning process be fallible because we have not yet reached a point in our evolutionary development where reason is perfected?
Here Nagel separates the basic operation of logic from the ambiguities of perception and judgment. Certainly our perception can evolve, just as our ability to see can evolve. But what about the basic rules of inference that come from pure logic? He uses the example of a person driving south in the morning, but notices that the sun is rising on his right. There is clearly a problem. Perhaps he is driving north; perhaps it’s not morning and the sun is really setting. In this case, pure reasoning raises a question about perception. So if reasoning is not reliable, how can it reliably correct perception?
The problem is compounded by the fact that we use such reasoning to think about evolution. The very process of creating “just so” stories about evolution, as Nagel calls them, is filled with logical inferences. If those logical inferences are based on a logic that is fallible, then, perhaps, our reasoning about evolution is flawed. But Nagel rejects this line of thinking as he should.
Nagel believes that our ability to do basic logic is fully formed and reliable. He prefers to use the word “emerge” to describe how it came to be. My explanation is a little different. When consciousness became sufficiently complex, it opened up a whole new mental world to us. This is the world of logic, of mathematics, of the conceptual models that are the basis for the laws of physics. Once this mental world became available to our consciousness, we could think abstractly about the world; we could reason about our environment; we could improve our lives by such reasoning; we could even focus consciousness on itself and reason about consciousness. I could say that reason “emerged” as consciousness became complex, but I prefer to say that we discovered a new tool, much like we discovered other important tools like clubs, spears and fire.
Nagel’s main criticism of materialistic reductionism still stands, however. If materialism cannot explain consciousness, then it cannot explain reason either. And if reason “emerges” as Nagel speculates, then that factor gives some weight to Nagel’s favorite approach to the evolution of consciousness. That approach is based on teleological factors in the laws of physics. Teleological factors are attributes of the laws of physics that push evolution towards the type of goals that will produce consciousness. He writes about that in the next section of his book.