Possibilianism is a philosophy which rejects both the diverse claims of traditional theism and the positions of certainty in strong atheism in favor of a middle, exploratory ground. The term was invented by Robbie Parrish, a friend of neuroscientist David Eagleman who defined the term in relation to his book of fiction Sum: Forty Tales from the Afterlives. Asked whether he was an atheist or a religious person on a National Public Radio interview in February 2009, he replied "I call myself a Possibilian: I'm open to...ideas that we don't have any way of testing right now." In a subsequent interview with the New York Times, Eagleman expanded on the definition:
"Our ignorance of the cosmos is too vast to commit to atheism, and yet we know too much to commit to a particular religion. A third position, agnosticism, is often an uninteresting stance in which a person simply questions whether his traditional religious story (say, a man with a beard on a cloud) is true or not true. But with Possibilianism I'm hoping to define a new position — one that emphasizes the exploration of new, unconsidered possibilities. Possibilianism is comfortable holding multiple ideas in mind; it is not interested in committing to any particular story."
In a New Yorker profile of Eagleman—entitled "The Possibilian"—Burkhard Bilger wrote:
Science had taught him to be skeptical of cosmic certainties, [Eagleman] told me. From the unfathomed complexity of brain tissue—"essentially an alien computational material"—to the mystery of dark matter, we know too little about our own minds and the universe around us to insist on strict atheism, he said. "And we know far too much to commit to a particular religious story." Why not revel in the alternatives? Why not imagine ourselves, as he did in Sum, as bits of networked hardware in a cosmic program, or as particles of some celestial organism, or any of a thousand other possibilities, and then test those ideas against the available evidence? "Part of the scientific temperament is this tolerance for holding multiple hypotheses in mind at the same time," he said. "As Voltaire said, uncertainty is an uncomfortable position. But certainty is an absurd one."
An adherent of possibilianism is called a possibilian. The possibilian perspective is distinguished from agnosticism in its active exploration of novel possibilities and its emphasis on the necessity of holding multiple positions at once if there is no available data to privilege one over the others. Eagleman has emphasized that possibilianism reflects the scientific temperament of creativity and intellectual humility in the face of "the known unknowns."