Pinker explains that in Part 3 he’ll defend Enlightenment ideals from various naysayers, from populists to left-wing intellectuals and cultural pessimists. He begins by examining a misconception about Enlightenment thinkers: They didn’t, as many believe, argue that humans are entirely rational but rather that we’re often irrational and therefore should rely on reason more often. Pinker claims that nowadays even those who prefer feeling to thinking, or believe that trying to rationalize with people is futile, tend to defend their arguments with personal reasons rather than data, undermining their point. Much evidence shows how people can be irrational, from availability bias to confirmation bias and stereotyping, but we can still employ reason when we want to.

Kant, Spinoza, Hume, and Smith argued that people “ought to be rational” (353) and employ free speech, empirical testing, and logic, no matter how difficult. Pinker explains that while we have other primal instincts, we’re “a cognitive species that depends on explanations of the world” (353). Although “vulnerable” to fallacies, humans routinely question and correct them. Pinker explains why humans are “so easily led into folly” (355). He denies that poor education is the main reason that people embrace irrational ideas.