Soul

The soul was, in religious belief, the spiritual essence of an individual, existing beyond the limits of the physical body.

In 2269, Leonard McCoy theorized that transporters actually created identical duplicates of people then destroyed the original body, and if so, wondered if the soul, if it existed, was duplicated as well. Montgomery Scott argued that since the soul was immortal by definition, then if it existed, it could not be destroyed. Spock later added that, since there was no known way to prove the existence of the soul, the question was meaningless. 

The Konor were a Samdian group who believed that those who did not possess their telepathic abilities were "Ikonor", or lacking souls, until 2365 when they were proven wrong by Data, who had been undergoing his own struggle with the question of whether or not he possessed a soul.