Enlightenment Now: The Case for Reason, Science, Humanism, and Progress

Nagel calls this line of thinking Cartesian, because it resembles Descartes’s argument “I think, therefore I am.” Just as the very fact that one is wondering whether one exists demonstrates that one exists, the very fact that one is appealing to reasons demonstrates that reason exists. It may also be called a transcendental argument, one that invokes the necessary preconditions for doing what it is doing, namely making an argument.³ (In a way, it goes back to the ancient Liar’s Paradox, featuring the Cretan who says, “All Cretans are liars.”) Whatever you call the argument, it would be a mistake to interpret it as justifying a “belief” or a “faith” in reason, which Nagel calls “one thought too many.” We don’t believe in reason; we use reason (just as we don’t program our computers to have a CPU; a program is a sequence of operations made available by the CPU).⁴

Though reason is prior to everything else and needn’t (indeed cannot) be justified on first principles, once we start engaging in it we can stroke our confidence that the particular kinds of reasoning we are engaging in are sound by noting their internal coherence and their fit with reality. Life is not a dream, in which disconnected experiences appear in bewildering succession. And the application of reason to the world validates itself by granting us the ability to bend the world to our will, from curing infections to sending a man to the moon.

Despite its provenance in abstract philosophy, the Cartesian argument is not an exercise in logic-chopping. From the most recondite deconstructionist to the most anti-intellectual purveyor of conspiracy theories and “alternative facts,” everyone recognizes the power of responses like “Why should I believe you?” or “Prove it” or “You’re full of crap.” Few would reply, “That’s right, there’s no reason to believe me,” or “Yes, I’m lying right now,” or “I agree, what I’m saying is bullshit.” It’s in the very nature of argument that people stake a claim to being right. As soon as they do, they have committed themselves to reason—and the listeners they are trying to convince can hold their feet to the fire of coherence and accuracy.

By now many people have become aware of the research in cognitive psychology on human irrationality, explained in bestsellers like Daniel Kahneman’s Thinking Fast and Slow and Dan Ariely’s Predictably Irrational. I’ve alluded to these cognitive infirmities in earlier chapters: the way we estimate probability from available anecdotes, project stereotypes onto individuals, seek confirming and ignore disconfirming evidence, dread harms and losses, and reason from teleology and voodoo resemblance rather than mechanical cause and effect.⁵ But as important as these discoveries are, it’s a mistake to see them as refuting some Enlightenment tenet that humans are rational actors, or as licensing the fatalistic conclusion that we might as well give up on reasoned persuasion and fight demagoguery with demagoguery.

To begin with, no Enlightenment thinker ever claimed that humans were consistently rational. Certainly not the über-rational Kant, who wrote that “from the crooked timber of humanity no truly straight thing can be made,” nor Spinoza, Hume, Smith, or the Encyclopédistes, who were cognitive and social psychologists ahead of their time.⁶ What they argued was that we ought to be rational, by learning to repress the fallacies and dogmas that so readily seduce us, and that we can be rational, collectively if not individually, by implementing institutions and adhering to norms that constrain our faculties, including free speech, logical analysis, and empirical testing. And if you disagree, then why should we accept your claim that humans are incapable of rationality?

Often the cynicism about reason is justified with a crude version of evolutionary psychology (not one endorsed by evolutionary psychologists) in which humans think with their amygdalas, reacting instinctively to the slightest rustle in the grass which may portend a crouching tiger. But real evolutionary psychology treats humans differently: not as two-legged antelopes but as the species that outsmarts antelopes. We are a cognitive species that depends on explanations of the world. Since the world is the way it is regardless of what people believe about it, there is a strong selection pressure for an ability to develop explanations that are true.⁷