I'm a six,” Freddy Lowell says after a thoughtful pause. Seven milligrams of Dilaudid, a potent narcotic pain reliever, is already being pushed into a port in his chest by a visibly busy nurse. She registers his pain score with a grunt. She will be back in two hours to ask for an updated number. At some point during this time, his pain will creep back, so strong it will render him mute. When the nurse next inquires, he will respond with seven fingers, his face buried in his pillow.

The Numerical Rating Scale has been a mainstay of clinical care since the late 1990s, when pain came to be referred to as the “fifth vital sign” alongside temperature, blood pressure, heart rate, and respiratory rate. But while the first four are measured with precise objectivity, pain is nuanced, complex, and personal, so much so that Freddy will quickly chastise anyone who tries to empathize with his plight. “Nobody knows what it's like,” he says decisively.