Sagan reiterates his position on extraterrestrial life—that the magnitude of the universe permits the existence of thousands of alien civilizations, but no credible evidence exists to demonstrate that such life has ever visited earth.[5] Sagan explores 15 billion years of cosmic evolution and the development of science and civilization.[6] He traces the origins of knowledge and the scientific method, mixing science and philosophy, and speculates about the future of science.[7] He also discusses the underlying premises of science by providing biographical anecdotes about many prominent scientists, placing their contributions in the broader context of the development of modern science.[8]