Figment

a figment of one's imagination

He had not seen sarcomeres: these segments were a figment of his imagination.

Perhaps, dear reader, you are only a figment in the dream of some god, as Sherlock Holmes was a figment in the mind of Sir Arthur Conan Doyle

Jung's implication here is clearly that one should try to forget that this is only a figment or fantasy, merely make-believe—or perhaps that one should forget the “only,” the “merely”—and indeed take the fantasy seriously as a reality.