Except in pure mathematics, nothing is known for certain (although much is certainly false).
Carl SaganWe have begun to contemplate our origins: starstuff pondering the stars; organized assemblages of ten billion billion billion atoms considering the evolution of atoms; tracing the long journey by which, here at least, consciousness arose.
Carl SaganThe Bill of Rights decoupled religion from the state, in part because so many religions were steeped in an absolutist frame of mind - each convinced that it alone had a monopoly on the truth and therefore eager for the state to impose this truth on others.
Carl SaganThe impediment to scientific thinking is not, I think, the difficulty of the subject. Complex intellectual feats have been mainstays even of oppressed cultures. Shamans, magicians and theologians are highly skilled in their intricate and arcane arts. No, the impediment is political and hierarchical.
Carl Sagan