The Parasite by Sir Arthur Conan Doyle
Being a physiologist, Austin Gilroy is unconvinced that the occult is real. His friend Professor Wilson, however, is not only convinced that psychical powers are real, but eagerly desires that Gilroy should be persuaded. To this end, Wilson invites Austin to his house for a demonstration. The effect is that Austin, although still skeptical, now concedes that there is more in the matter than he at first believed. But when the psychic, Miss Penclosa, controls his actions to the point where he nearly murders his fiancee, Austin Gilroy doubts no longer.