Perhaps the best way to think of what makes a human being human is to look at something that seems almost human and subtract the difference. Whatever is left over is what is unique to us. That seems to be the thinking behind the Finnish photographer Maija Tammi’s One of Them Is a Human #1, a portrait of Erica, the Japanese android who was declared the most realistic female human robot of 2016.

Astrophysicists, art critics, biologists, and tennis fans seek to define the indefinable. By Patrick House.