As science has become more specialized and more costly, its questing spirit has been constrained by dogma. And philosophy, perhaps the discipline best placed to question orthodoxy, has retreated behind dense theoretical language and arcane topics of learning. Sagan draws on classical and contemporary philosophy to intervene provocatively in often-charged debates on thermodynamics, linear and nonlinear time, purpose, ethics, the links between language and psychedelic drugs, the search for extraterrestrial intelligence, and the occupation of the human body by microbial others.