Do You Only Have a Brain? On Thomas Nagel
This style of argument does not, alas, have a promising history. After all, what could be more common-sensical, obvious or evident than the notion that the earth is flat and the sun revolves around the earth? All ordinary evidence supports that verdict: we know from experience that people fall off things that are spherical, especially when trying to hang upside down from them, and we know that the sun rises in the sky in one direction and sets in the other as it revolves around the seemingly flat earth. Happily, Nagel does not attempt to repudiate the Copernican revolution in astronomy, despite its hostility to common sense. But he displays none of the same humility when it comes to his preferred claims of common sense—the kind of humility that nearly 400 years of nonevident yet true scientific discoveries should engender. Are we really supposed to abandon a massively successful scientific research program because Nagel finds some scientific claims hard to square with what he thinks is obvious and “undeniable,” such as his confidence that his “clearest moral…reasonings are objectively valid”?
In support of his skepticism, Nagel writes: “The world is an astonishing place, and the idea that we have in our possession the basic tools needed to understand it is no more credible now than it was in Aristotle’s day.” This seems to us perhaps the most startling sentence in all of Mind and Cosmos. Epistemic humility—the recognition that we could be wrong—is a virtue in science as it is in daily life, but surely we have some reason for thinking, some four centuries after the start of the scientific revolution, that Aristotle was on the wrong track and that we are not, or at least not yet. Our reasons for thinking this are obvious and uncontroversial: mechanistic explanations and an abandonment of supernatural causality proved enormously fruitful in expanding our ability to predict and control the world around us. The fruits of the scientific revolution, though at odds with common sense, allow us to send probes to Mars and to understand why washing our hands prevents the spread of disease. We may, of course, be wrong in having abandoned teleology and the supernatural as our primary tools for understanding and explaining the natural world, but the fact that “common sense” conflicts with a layman’s reading of popular science writing is not a good reason for thinking so.
Incompatibility with common sense is not Nagel’s only argument against naturalism. A second line of argument begins by appealing to what he takes to be an everyday opinion: that there are objective moral, logical and mathematical truths. He then argues that the existence of these kinds of objective truths is incompatible with naturalism. For the moral case, Nagel asks: If our moral faculties are simply the result of evolution, how can they be reliable measures of objective moral truth? Why should evolution prefer the perception of moral truth to whatever happens to be advantageous for reproduction? Thus, if some of our moral beliefs really are objectively true, then they cannot be the result of evolution. And because he is confident that we do know some objective moral truths, Nagel concludes that “a Darwinian account of the motives underlying moral judgment must be false, in spite of the scientific consensus in its favor.” Recognizing that readers will find this inference jarring, Nagel adds: “I, even more strangely, am relying on a philosophical claim to refute a scientific theory supported by empirical evidence.”
There is, indeed, much that is strange here. To begin, there is nothing remotely common-sensical about Nagel’s confidence in the objectivity of moral truth. While Nagel and his compatriots apparently take very seriously their moral opinions—so seriously that they find it incredible to suggest that their “confidence in the objective truth of [their] moral beliefs” might, in fact, be “completely illusory”—this can hardly claim the mantle of “the common sense view.” Ordinary opinion sometimes tends toward objectivism, to be sure—often by relying on religious assumptions that Nagel explicitly rejects—but it also often veers toward social or cultural relativism about morality. Whether morality is truly objective is a philosopher’s claim (and a controversial one even among philosophers) about which “common sense” is either agnostic or mixed.
We take no stance on Nagel’s hypothesis that if our moral faculties are simply the result of evolution, they cannot be reliable measures of objective moral truth. But we should note that Nagel’s colleague, philosopher Sharon Street, accepts it and draws the opposite conclusion. She argues that because this hypothesis is true, and because we are obviously the products of evolution, we should give up the idea that there are objective moral truths in Nagel’s sense. Given the philosophical plausibility of Street’s alternative response—not to mention the simplistic evolutionary reasoning the whole debate is predicated on—it is hard to see why any biologist should be given pause by Nagel’s argument.
A more interesting challenge—really, the only interesting philosophical point raised in the book—concerns logical and mathematical truths. Is it possible, Nagel asks, to reconcile a naturalistic and biological picture of the evolution of our cognitive capacities with the confidence we have in our ability to do logic and mathematics? Nagel’s argument invokes a contrast with our perceptual capabilities, because our ability to reliably perceive many of the features of our physical environment seems likely to have an evolutionary explanation. (After all, if we could not reliably spot sudden cliffs or saber-toothed tigers, our reproductive fitness would be seriously compromised!) But logical truths are not like that, Nagel argues. It is self-evident that something cannot be both red and not-red at the same time (the “law of non-contradiction”). So, too, it is self-evident that if all men are mortal, and Socrates is a man, then Socates is necessarily mortal. Even if evolution endowed us with the capacity to recognize the law of non-contradiction and to draw valid deductive inferences, how does it explain the obvious truth of these logical claims? Nagel’s response to this question is that evolution cannot—and the problem is even worse than that:
Any evolutionary account of the place of reason presupposes reason’s validity and cannot confirm it without circularity.
Eventually the attempt to understand oneself in evolutionary, naturalistic terms must bottom out in something that is grasped as valid in itself—something without which the evolutionary understanding would not be possible.
In other words, even if one thinks there is an evolutionary explanation for why we recognize the obviousness of logical, mathematical and scientific truths, there would still be the question of why we think evolutionary theory itself is justified. An evolutionary explanation of that latter fact would have to presuppose the correctness of the theory whose justification we are questioning, making the argument circular: we would have to assume that evolutionary theory is true in order to investigate whether it is true!
* * *