“Neanderthals were prone to depression, he said. He said they were prone to addiction, too, and especially smoking.” So launches Rachel Kushner’s fourth novel, Creation Lake, which vacillates between two unlikely philosophical bedfellows: Sadie Smith, an American private spy with “banal” beauty and a made-up name, and Bruno Lacombe, an octogenarian hermit whose emails she hacks to infiltrate Le Moulin, a commune of his leftist acolytes in rural France.

As we learn early on, Sadie is hardly heading to Vantome to sleuth out political solidarity. What she gathers from Bruno’s lengthy lectures about the legacy of French leftism can, and will, be used against his group of radicals. “Part of my job is to be something of an expert on such events and the social movements that precipitate them,” Sadie relays. She is quick to assert that “none of these eruptions . . . resulted in the overthrow of capitalism . . . not a single one” but later surmises, more wistfully, that “maybe it is only by admitting that some harmful condition is permanent, that you begin to locate a way to escape it.”

At first, Creation Lake seems like Kushner’s most cerebral and ruminative book to date. But Sadie’s intellectual investigation of the Moulinards’ practices may also be read as the tale of a cynic who gradually, stubbornly, begins to believe again. As usual, Kushner flouts prescriptions for how to build a better world and instead exalts the ability to creatively endure, and know, the one we have already.