Most people imagine philosophers as rational thinkers who spend their time developing abstract logical theories and strongly reject superstitious beliefs. But several 20th-century philosophers actively investigated spooky topics such as clairvoyance, telepathy – even ghosts.

Many of these philosophers, including Henri Bergson and William James, were interested in what was called “psychical research”. This was the academic study of paranormal phenomena including telepathy, telekinesis and other-worldly spirits.

These thinkers attended seances and were attempting to develop theories about ghosts, life after death and the powers exhibited by mediums in trances. My recent archival research has been looking at how these topics shaped 20th-century philosophy...