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Charles Darwin
English naturalist who introduced an influential version of the theory of evolution in his 1859 On the Origin of Species. Darwin held that the development of any species can be traced to competition within and between species in which the fittest survive. At one time Darwin was clearly a theist, and in his public writings he talked of "laws imprinted on matter by the Creator." But in later life, he became an agnostic and possibly an atheist, writing that "the whole subject is too profound for the human intellect"; some have said that the premature death of his daughter had a devastating effect on his early faith.

Paul Davies
Theoretical physicist at the University of Newcastle upon Tyne and author of numerous works on modern science ranging from About Time and Superforce to God and the New Physics and The Mind of God.

Richard Dawkins
Professor of the Public Understanding of Science at Oxford University. He is the best-known contemporary Darwinist. His theory of the selfish gene is an influential restatement of evolutionary theory. Works include The Selfish Gene, The Extended Phenotype and The Blind Watchmaker.

Democritus
Democritus of Abdera (c. 460-370 B.C.), co-founder of the theory of atomism, held that the only reality was matter. He denied the existence of any mental reality or deity separate from matter. Democritus held that the world was made up entirely of an infinite number of tiny atoms that have been in random motion for all eternity.

Daniel Dennett
Modern defender of materialism, the rejection of any mental reality separate from the material. In his Consciousness Explained, he argues that consciousness is nothing more than "the mechanistically functional organization of a physical system."

Rene Descartes
Descartes (1596-1650) was the French mathematician/philosopher who is sometimes said to have laid the foundations of modern philosophy by starting his inquiry with the question of how knowledge can be validated ("Cartesian skepticism"). He was a critic of scholasticism and developed his own theory of a mind separate from the brain ("Cartesian dualism").

David Deutsch
Contemporary exponent of the Many Worlds interpretation of quantum physics. He is the author of The Fabric of Reality: The Science of Parallel Universes - And its Implications.

Paul Dirac
Quantum physicist who first predicted the existence of anti-matter. His transformation theory showed that Heisenberg's matrix mechanics and Schroedinger's wave mechanics models of quantum were mathematically equivalent.