Einstein, Albert -- Lecture (1933-06-10), “On the Method of Theoretical Physics,” Herbert Spencer Lecture, Oxford

Dave — Sun, 01 Feb 2004 16:00:00 +0000

It can scarcely be denied that the supreme goal of all theory is to make the irreducible basic elements as simple and as few as possible without having to surrender the adequate representation of a single datum of experience.

Albert Einstein (1879-1955) German-American physicist
Lecture (1933-06-10), “On the Method of Theoretical Physics,” Herbert Spencer Lecture, Oxford
(Source)

Often shortened down, as "Einstein's Law," to something like "Make everything as simple as possible, but not simpler" or "Everything should be made as simple as possible, but not simpler."

This in turn appears to first come from an article "How a 'Difficult' Composer Gets That Way," by Roger Sessions, New York Times (1950-01-08), where he says (clearly paraphrasing):

I also remember a remark of Albert Einstein, which certainly applies to music. He said, in effect, that everything should be as simple as it can be but not simpler!

Shortly after that, other references were made elsewhere treating the phrase as an actual quotation by Einstein.

For more about this quotation and its origin, see:

Quote Origin: Everything Should Be Made as Simple as Possible, But Not Simpler – Quote Investigator®.
Alice Calaprice, ed., The Quotable Einstein, "Attributed to Einstein" (2005 ed.).

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Einstein, Albert -- Lecture (1933-06-10), “On the Method of Theoretical Physics,” Herbert Spencer Lecture, Oxford