Unobtrusive Measures

You can measure height, weight and strength. Samuel did exactly that when he paid a visit to Jesse's sons. But how do you measure character?

Fitbit, Garmin, Misfit, Jawbone, Huawei, Moov, Polar, TomTom, Withings, Xiaomi.

These are all brand names of devices collectively known as "activity trackers" and "wearable technology." Some of them also have apps you can install in your smartphone to track certain activities, such as how many steps you take each day, what distance you ride your bicycle, how many calories you consume and what your heart rate is at any given moment.

David Wolpe, a rabbi at Sinai Temple in Los Angeles, upon learning from his daughter that his Apple iPhone would count his steps, says that information gave him "an entirely new field to uselessly obsess over." He says he now not only checks his steps, but also wonders whether he should have more steps and worries on the Sabbath about how many steps he is "missing" since he does not carry his phone on that day.

But all that led him to think about things that cannot be measured. "There is no scale for tenderness or affection," he says, writing on his...