This passing shadow

"And because they are so sure of what they think they know, it will never occur to them to ask him, Doctor, is the palm tree a tree. One day they will go their separate ways and the fundamental question of whether the palm tree is a tree because it resembles a tree, or whether this passing shadow we cast on the ground is life because it resembles life, will remain unanswered."

- Jose Saramago, The Year of The Death of Ricardo Reis (298)