Why do so many people have an immediate, intuitive grasp of this highly abstract concept—“subjective age,” it’s called—when randomly presented with it? It’s bizarre, if you think about it. Certainly most of us don’t believe ourselves to be shorter or taller than we actually are. We don’t think of ourselves as having smaller ears or longer noses or curlier hair. Most of us also know where our bodies are in space, what physiologists call “proprioception.”