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." 
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."
According to the Dallas Morning News and MSNBC, the possibilian concept—including various spellings (e.g. "possibillion") and modifications (e.g. "possibilitarian")—has become popular on the internet. By November 2009, The List Magazine wrote: "Googling 'possibilian', the position Eagleman invented to explain his belief system, throws up the beginnings of a worldwide movement."
In an article in the New Statesman, the atheist author Philip Pullman declared himself a possibilian, as did Wired magazine founding editor Kevin Kelly in an interview in the LA Times. By April 2011, "close to a thousand Facebook members had switched their religious affiliation to Possibilianism."
Sam Harris (a new atheist) has attacked possibilianism as "intellectually dishonest", and its description of strict atheism as a straw man. Harris writes that the position Eagleman espouses "is, simply, atheism." Harris calls on Eagleman "to admit that “possibilianism,” this middle position of yours, is just a piece of performance art, rather than a serious thesis." In response, Eagleman stated that "[Harris'] braggadocio appears to be emblematic of the neo-atheist posture, and confirms why I don't feel completely at home in that camp." Journalist Steve Volk in the Huffington Post suggested that Harris and Eagleman should be "new allies": "If we're going to get beyond the typical exchanges between new atheists and the religious, I'd argue that it's through figures like Eagleman and Harris that we will find the most productive path: men who are eager to use science while demonstrating a capacity to consider ideas from other areas of human experience and systems of thought."
- Theological noncognitivism
- Logical positivism
- Naturalism (philosophy)
- Beyond God and atheism: Why I am a possibilian, David Eagleman, New Scientist, Sep 27, 2010.
- Envisioning the Afterlife, interview with David Eagleman on NPR's On Point, Feb 27, 2009.
- Stray questions for David Eagleman, New York Times Paper Cuts, July 10, 2009.
- Neuroscientist Imagines 40 Different Versions of the Afterlife, KPBS interview with David Eagleman, Mar 16, 2009.
- The Soul Seeker: A neuroscientist's search for the human essence, Texas Observer cover story, June 3, 2010.
- NPR Talk of the Nation: Interview with David Eagleman, Feb 17, 2009.
- The Possibilian: David Eagleman and the Mysteries of the Brain, The New Yorker, Apr 25, 2011.
- "I'm a possibilian", Rick Kleffel's The Agony Column, Mar 13, 2009.
- Are you surrounded with authentic communities?, The Huffington Post, 1 March 2009.
- Choose your afterlife, MSNBC.com, Sept 10, 2009.
- Ideas for modern living: uncertainty. The Observer (UK). 9 May 2010.
- Why I am a Possibilian, TEDx talk by David Eagleman, Oct 2010.
- Lanham, F. Writing about what comes next. Houston Chronicle. 16 Feb 2009.
- Houston author stunned by buzz over 'possibilian' theory, Dallas Morning News, June 16, 2009.
- Choose (after)life. The List Magazine, Issue 643, 4 November 2009.
- You could be a Possibilian..., The Daily Monitor, Uganda. Jan 10, 2010.
- Beyond Good and Bad, Economic Times of India, July 29, 2010.
- Philip Pullman on what he owes to the Church of England, New Statesman, 9 June 2011
- Patt Morrison Asks: The Possibilian, Kevin Kelly, 13 Aug 2011
- Whither Eagleman?, Sam Harris, 2011.
- Eagleman Blog: Why I am a possibilian, David Eagleman, 2012.
- New Allies In The Theist/Atheist Debate, Steve Volk, Huffington Post, 8/25/2011.
- Pop!Tech talk on possibilianism
- The possibilium
- Lecture: On Uncertainty, delivered at the School of Life, London, UK, 23 May 2010.
- Beyond God and atheism: Why I am a possibilian, David Eagleman, New Scientist, 27 September 2010.
- The Possibilian: David Eagleman and the Mysteries of the Brain, The New Yorker, 25 April 2011.