If we want to solve a problem that we have never solved before, we must leave the door to the unknown ajar.

Richard Feynman (1918-1988) American physicist

Speech (1955-11), “The Value of Science,” National Academy of Sciences Autumn Meeting, California Institute of Technology, Pasadena
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Reprinted in What Do You Care What Other People Think? (1988).

Feyman used this general construction on multiple occasions, e.g., in a speech (1964-09), "What Is and What Should Be the Role of Scientific Culture in Modern Society," Galileo Symposium, Florence, Italy (reprinted in The Pleasure of Finding Things Out, ch. 4 (1999) [ed. Jeffrey Robbins]):

In order to make progress, one must leave the door to the unknown ajar -- ajar only.