I like the general idea that “we should not believe anything that cannot be scientifically proven.” God, ghosts and Santa Claus (for that matter) are ideas that have no scientific proof. However, given the definition of “atheist” I suppose there is no reason an atheist could not believe in ghosts – or Santa Claus.

Humans once thought the earth was flat. Science corrected that. The same thing will happen with ghosts. I have seen an apparition. As an Agnostic my expectation is some version of the Quantum Multiverse (science) will explain it in the future.