There is a universal Principle of Reversibility which dictates that anything we cannot see or detect via one of our senses, by the same token cannot see or detect us. Thus microbes, thus the future, thus stars situated beyond our event horizon, thus the interior of any person who comes by and greets us, thus dead people, all of them, 100 percent of them. When we watch a film we do not see it—given that the characters cannot see us. But to fully understand the rule, we have to imagine it’s as if the norm were that children didn’t look anything like their parents, so that they wouldn’t recognize themselves in them. Not easy. But essential to understand.