It must be remembered that evidence is never complete, that knowledge of truth is always partial, and that to await certainty is to await eternity.

John Bowlby 1907-1990) British psychologist, psychiatrist, psychoanalyst
Maternal Care and Mental Health (1951)

The last phrase is often attributed to Jonas Salk, who used it ("It is said to await certainty is to await eternity") in a telegram to Basil O'Connor (8 Nov 1954). But as Salk himself noted, it was not original to him.