At the beginning of his popular Cosmos television series scientist and author Carl Sagan declared that “the universe is all that is, all there ever was and all there ever will be.” Sagan was a proponent of naturalism, the belief that nothing exists beyond the world available to our senses. There is no God, and hence no creative source of the world's existence other than nature itself. Nature, a term derived from the Latin word “to give birth,” is self-generating. Nature is quite enough all by itself, and religions professing belief in God or gods are fictitious distractions at best.

To its adherents naturalism is not only intellectually satisfying but also emotionally liberating. It is a breath of fresh air in a world made stale by the obsessive recitations of religion. Naturalism boldly turns our attention toward the immensities of this world even as it embeds us within the cosmic processes that gave birth to life. It rescues adventurous souls from what they take to be the backwardness, irrelevance and oppressiveness of traditional forms of spirituality. Naturalism also has the advantage – or so it would seem – of being completely reconcilable with science.

Naturalism comes in many flavors, but the focus here will be on the specific variety known as scientific naturalism. Scientific naturalism assumes not only that nature is all there is but also that science is the only reliable way to understand it.