In 2011, speculative CBS thriller Person of Interest began with the premise that a computer genius named Finch (played by Michael Emerson) had built an AI that could tell when acts of violence were about to occur in New York City. In order to intervene, he hired a former special operative named Reese (Jim Caviezel). The only information that the AI ever provides is a social security number; each episode, this leaves Finch and Reese to solve the central question: does this SSN belong to a victim of an impending crime, or a perpetrator? Do they need to be stopped, or saved?

Solving this question typically involves a veritable cornucopia of guesswork, research, hacking, plot twists, and pieces of paper taped to walls and connected by webs of string (one of my favorite TV tropes, featured in Supernatural and It’s Always Sunny in Philadelphia). Chess also features prominently, as do various secretive systems of communication. No episode is quite as likely to appeal to your inner solver, though, as season four, episode two, “Nautilus,” which aired on September 30^th, 2014.

In this episode named for its chambered nautilus shell motif, Finch and Reese receive the SSN for chess grand master and college student Claire Mahoney (Quinn Shephard). She’s AWOL from school and chess in pursuit of a mysterious game’s prize. The game began with a post on an obscure message board: an image of a nautilus, captioned, “If you seek enlightenment, be the first to walk through the chambers.” There was data hidden in the image of a nautilus, and it sent Claire off down the rabbit hole.

Following her, Finch watches as she pulls a tab of paper from a lost dog sign, with a nautilus watermarked behind the numbers. After calling the number leads to nothing, Finch realizes: “It’s not a phone number . . . it’s multiplication.” Multiplying produces GPS coordinates, pointing to a location in Harlem.

There, a mural of seemingly random shapes (painted by artist Apache Gonzalez) covers a wall. Once again, there’s the nautilus. Back in his office, Finch studies a photo of the mural, finally recognizing it as a variation on a Bongard Problem. He explains, “This particular type of puzzle presents two sets of diagrams. The diagrams in the first set share a common feature. The blocks never overlap with the curved lines. Conversely, in the second set, the blocks do touch the curves, but there’s another common element.”

Reese interjects, “There’s a different number of blocks in each diagram.”

“Using this pattern,” Finch continues, “I can fill in the blank space with the only number of blocks left out, which is three, thus solving the puzzle, creating a sort of three-pronged arch.” Googling, Reese finds a matching photo of an arch in Central Park.

That night, Claire is near the arch, standing in traffic. When Reese interrogates her about her apparent death wish, they’re interrupted, but later, the answer comes to light. If you stand at the right point in the street, banners for “motorcycle safety month” visually blend together to show the faint image of a nautilus. Beneath the nautilus: pictures of traffic lights that Finch correctly identifies as the equivalent of Braille dots. They spell out, “184^th and 3^rd.”

Claire’s next found in a biker bar at 184^th and 3^rd, staring at a bulletin board decorated in gang logos. One features a skull with nautiluses for eyes; letters surround the skull in a seemingly random arrangement. At an otherwise dead end, Reese sits at his desk, rewriting the letters over and over, seeking a scrambled word, until Fusco (Kevin Chapman) determines that the letters refer to musical notes, forming the tune to “New York, New York.” Reese is able to deduce a location: the Empire State Building’s observation deck.

Here on out, the puzzles become simpler, less compelling; from a Doylist standpoint (referring to author Sir Arthur Conan Doyle’s perspective, as opposed to fictional character John Watson’s), we might assume that the writers have decided that it’s time for the plot to dominate, turning the puzzles into mere perfunctory means to an end. I’m convinced, however, by the Watsonian explanation, that Claire has sufficiently proven herself, and the AI game-master is content now to lead her by the hand.

But why the nautilus shell? In the episode, computer hacker Root (Amy Acker) refers to the nautilus as one of nature’s examples of a logarithmic spiral. It’s commonly also referred to as an example of the golden ratio, but as explained in “Math as Myth: What looks like the golden ratio is sometimes just fool’s gold,” that’s not so true. Is that they key—that the prize Claire seeks is fool’s gold?

The episode’s primary puzzles have been solved, and the series has come to an end (though it can be streamed on HBO Max). Still, this one question of the nautilus’ significance remains. What is the symbolic connection between a nautilus shell’s chambers and the “enlightenment” the game promises? Is enlightenment encoded in the logarithmic spiral, or in something more particular to the mollusk itself? I don’t have the answers, but as the nineteenth-century poem “The Chambered Nautilus” by Oliver Wendell Holmes Sr. shows, Person of Interest’s screenwriters were not the first to see something spiritual in the nautilus, and I doubt they’ll be the last.

In closing, I offer the following excerpt from Melissa Febos’ Body Work: The Radical Power of Personal Narrative to mull over in your own pursuit of the nautilus shell’s deeper meaning:

The spiral does not belong to the nautilus shell, unless it also belongs to the whirlpool, the hurricane, the galaxy, the double helix of DNA, the tendrils of a common vine. If there are golden ratios that govern the structures of our bodies and our world, then of course there must be such shapes among the less measurable aspects of existence.

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