Know thyself. It’s an old adage that has new resonance in the digital age. Today, you can buy smart devices that monitor your heartbeat, blood pressure, exercise habits, water intake, sleep, mood, menstrual cycle, sexual activity, and meditation patterns, not to mention your poop. The internet of things has turned into what academic and author Andrea Matwyshyn has termed the “Internet of Bodies” with the promise of selling you insights about your “quantified self.” The desire for self-awareness is not new, but these data offer a dif­ferent twist on enlightenment. Millions of Americans live with a smartwatch that reminds them to stand, breathe, and take a few more steps to meet their daily exercise goals. This helpful (and healthful) algorithmic prompt only works, of course, because your smart device is tracking your bodily activity. It literally knows you are breathing, which can be helpful to police if for some reason you stop. The data we produce—from our step count to our DNA—is increasingly coming under surveillance. Not all of this surveillance is unwelcome. Many medical …