That doesn’t mean we always use language to convey reality. Language is a social medium with social purposes. Sometimes, we use it not to communicate facts about the world but to filter them. We euphemize bribes as “contributions” to preserve the dignity of lobbyists and legislators. We phrase treaties vaguely because if they were clear, nobody would sign them. We invent subtle sexual overtures to avoid a confrontation if the other guy turns out not to be gay. We complain about doublespeak but rely on double meanings.

These are the aspects of our duality: brain and mind, matter and metaphor, fact and frame, science and politics, information and implication. Even their common lesson has two sides. On the one hand, we must face the limits of our mental construction. We have trouble understanding intellectual property because our ideas of possession and theft are based on physical objects. We have trouble with evolution because we think of adaptation as something that individuals do in their lifetimes, not something a species does over generations. We confuse differences in group averages with claims of group superiority. We’re prone to cronyism because our notions of community arose from family and tribe. In criminal trials, we resist objective explanations of subjective behavior. In sum, Pinker warns, “the machinery of conceptual semantics makes us permanently vulnerable to fallacies in reasoning.”

On the other hand, we are not imprisoned by them. The dialectic of creativity and reality-testing has taken us far beyond other animals and can take us farther. The next step is to dump our most natural and mistaken metaphor — education as the filling of empty minds — and recognize that we learn by extrapolating, testing, modifying and recombining mental models of the world.

That’s the two-faceted human nature Pinker wants to show us through the window of language. But as he does so, one more face appears in the glass: the reflection of the man looking into it.

Being a scientist is hard. You’re supposed to keep your personality out of the way, justifying every topic of interest by some larger theoretical goal. Pinker tries. “I like to think I have a better reason to introduce you to my little friends,” he pleads, referring to verbs and his infatuation with them. But as Pinker’s little friends consume the book, it becomes clear that he’s a geek.

It starts on the first page. The book is pegged to the anniversary of Sept. 11, and that’s the first topic Pinker addresses. Here is Pinker’s angle: Was it one “event” or two? This question makes a $3.5 billion difference to the World Trade Center’s owner and his insurance company, but you’d be hard pressed to think up a more pointy-headed question about the murder of nearly 3,000 people. The riffs continue: verb taxonomies, the nuances of “politeness theory,” the comparative languages of South American tribes. At one point, Pinker draws up a game-theoretic matrix for the question “Would you like to come up and see my etchings?” Etchings, of course, are code for sex. But in Pinker’s case, you get the feeling that this guy actually would prefer to show you his etchings. That’s his kink. He’s interested in the stuff of life, but he’s even more interested in how we depict it.