I apologize if this is a duplicate, for I cannot find it with the search bar:

What are the facts? Again and again and again — what are the facts? Shun wishful thinking, ignore divine revelation, forget what "the stars foretell," avoid opinion, care not what the neighbors think, never mind the unguessable "verdict of history" — what are the facts, and to how many decimal places? You pilot always into an unknown future; facts are your single clue. Get the facts!

Time Enough for Love (1973) or The Notebooks of Lazarus Long (1978), Robert A. Heinlein