19 – What is a ghost?

Depends on the ghost. Sometimes memories can seem more real than the present world. Sometimes the mind projects on disparate stimuli a coherent interpretation that appears to be human, or some other intelligent agent. One can raise here, and for other questions in this list, all sorts of interesting psychological ruts that predispose the mind to see 'ghosts'.

We see faces everywhere (:0) because seeing and interpreting faces is really quite an important skill. We project patterns onto randomness, infer causes from consequences and are apt to attribute them to the actions of free-willed agents. We have a predisposition to infer malicious intent in the actions of others, particularly strangers. When left alone for lengthy periods most people, at some point get the feeling that they are not alone. At times of stress, even mild stress, the mind retreats to a more defensive, one might say primitive, position

The list goes on and on and one can dream up evolutionary explanations for all of these things. My guess is that ghosts are the false positives if primitive survival mechanisms trying to make sense of the world.