"The Value of Science". Public address at the National Academy of Sciences, published in "What Do You Care What Other People Think" (1988), Autumn 1955.
"The Character of Physical Law". Richard P. Feynman's Messenger Lectures at Cornell University (November 1964), later quoted in his book "The Character of Physical Law" (Chapter 1 "The Law of Gravitation", p. 13), www.youtube.com. 1965.