He shakes his little head, taps my fingers, and repeats forcefully, “Yellow.” He points to the list of numbers I’ve drawn with a long twig in the sand by the side of the Bassin de la Villette, not far from some old men playing pétanque. Then Sam points to the eight and says, “Gleen,” his word for green, and to the five and says, “Plue.” The six is red, the seven light green, the three bluish-yellow, and the two gray-red. He doesn’t like the figure one at all.

“Aha. So what is yellow plus yellow?”

“Gleen,” he answers straightaway, pointing to the figure eight. He loves eights. He loves dark green.

All that day we play the “numbers are colors” game, and by that evening, I’ve realized that not only does my son see numbers in color, but he also assigns sounds to colors and numbers, or characteristics such as friendliness or strength or nastiness. The sound of an Underground train arriving in a station—“wy-elv,” meaning “white eleven”—or of your blood pounding inside your head when you put your hands over your ears—“Plue-dweam, Daddy!,” meaning a blue dream. I also know that Sam assigns colors to the emotions he detects in other people. Objects too, although he cannot tell me which objects. He points to shadows and puddles, and I reckon I’d be happy to spend my entire life learning his language.

He’s very pale and quiet as we walk across the Place de la Bastille, clutching my hand and trundling along on his little chubby legs. I can feel the sun’s warmth on my back.

“It’s red and white here,” says Sam, although he doesn’t just say it, he gestures at walls and columns that have stood for a hundred years or more. “Ow, loud!” he says, his wise old eyes filling with tears.

That’s the day I finally begin to understand him properly. My son has more sensory receptors than other people, and so he suffers a constant bombardment of impressions that normal people don’t even notice on the edges of their perception. He’s a synesthete. I decide to break the news to Marie-France as gently as possible. However, Sam’s going to need a lot of courage and determination to cope with this gift and this “extra world.”

I search for a route home that I’m pretty sure doesn’t take us past too many corners, squares, and windowsills where attacks, murders, riots, suicides, or lynchings have taken place. Along the way, I buy Sam a large salted-butter caramel ice cream of the kind we used to eat back home in Finistère, at the tip of Brittany. As he licks at it, I promise him and myself that I’ll do everything I can to help him to survive in this extra world.