"In the 21st century, human nature has discovered a new mirror. Thanks to the rise of smartphones and wearables, such as the Apple Watch, we can now monitor our body and behavior in exquisite detail, tracking everything from the amount of sleep we get to the number of calories we swallow. We can count our steps and resting pulse, compare our social network and spending habits. (There's even a smartphone app that can tell you how often you check your smartphone.) Only a few years before, quantifying these aspects of life would have required a trained team of professionals. Now we can do it with the slim computer in our pocket. Unfortunately, it's not clear that this new feedback is helping us make better decisions. ... These new digital mirrors are certainly cool. I want to make them useful."