Why Does the World Exist: An Existentia - By Jim Holt Page 0,81

an endowed series of lectures on his contributions to theoretical physics. I went home and called his publicist at Oxford University Press to see if an interview could be arranged. A couple of days later, she rang back to say that “Sir Roger” had agreed to make a little time for me to talk about philosophy.

As it happened, Penrose was being put up in a stately apartment building fronting the west side of Washington Square, just steps away from my own Greenwich Village residence. On the appointed day, I set out across the square, which, given the glorious spring weather, was a typically cacophonous buzz of activity. Here, a pick-up jazz combo played for people lounging in the grass; there, a would-be Bob Dylan was wailing away over his guitar. By the big fountain in the middle of the square, banjee boys did improvised gymnastics for an earnest-looking audience of European tourists, while the dogs in the nearby dog run capered and barked.

I exited the square at the northwest corner, where the chess hustlers congregate over the outdoor chess tables, waiting for naive passersby to join them in a game and lose a little money. Glancing up at the old Earle Hotel near the corner, I recalled reading somewhere that it was during a stay in that very hotel many decades ago that the Mamas and the Papas had written their hit song “California Dreamin’.” Inevitably, that was the tune running through my head as I entered the lobby of the building where Penrose was staying, which was vaguely Moorish in decor. The liveried doorman told me to take the elevator to the penthouse.

Sir Roger himself answered the door. He was an elfin man who, with his thick head of auburn hair, looked much younger than his years. (He was born in 1931.) The apartment where he was staying was quite grand, in a prewar New York sort of way. Its high ceilings were embellished with elaborate moldings, and great casement windows with leaden mullions overlooked the treetops of Washington Square. By way of small talk, I pointed out an enormous elm, reputed to be the oldest tree in Manhattan, and told Sir Roger that it was known as the “hanging tree,” since it had been used for executions in the late eighteenth century. He nodded genially at this bit of unsolicited information, then padded off to the kitchen to fetch me a cup of coffee.

Why, I briefly wondered as I took a seat on the sofa, did everyone but me seem to find caffeinated beverages more conducive than alcohol to pondering the mystery of existence?

When Sir Roger returned, I asked him whether he really believed in a Platonic world, one that exists over and above the physical world. Wouldn’t such a two-world view be a little profligate, ontologically speaking?

“Actually, there are three worlds,” he replied, warming to my challenge. “Three worlds! And they’re all separate from one another. There’s the Platonic world, there’s the physical world, and there’s also the mental world, the world of our conscious perceptions. And the interconnections among these three worlds are mysterious. The main mystery I’ve been addressing, I suppose, is how the mental world is related to the physical world: how certain types of highly organized physical objects—our brains—seem to produce conscious awareness. But another mystery—which, to a mathematical physicist, is just as deep—is the relationship between the Platonic world and the physical world. As we search for the deepest possible understanding of how the world behaves, we’re driven to mathematics. It’s almost as though the physical world is built out of mathematics!”

So he was more than a Platonist—he was a Pythagorean! Or he was at least flirting with Pythagoras’s mystical doctrine that the world was constituted by mathematics: all is number. Yet I noticed that there was one link among his three worlds that Penrose hadn’t yet addressed. He had mentioned how the mental world might be linked to the physical world, and how the physical world might be linked to the Platonic world of abstract mathematical ideas. But what about the supposed link between that Platonic world and the mental world? How are our minds supposed to get in touch with these incorporeal Platonic Forms? If we are to have knowledge of mathematical entities, we have to “perceive” them in some way, as Gödel put it. And perceiving an object usually means having causal commerce with it. I perceive the cat on the mat, for example, because photons emitted from the