Einstein's complex general theory of relativity was accessible only to professional colleagues when it was first published in 1916. For Nature four years later (1920), Einstein sought to recount, for the general reader, the process by which he reached his revolutionary conclusions.
"On the occasion of the finding of the gravitational curvature of light rays by the British expedition that was sent to observe the eclipse of the sun, I have been urged by many to give a brief description to non-mathematicians of the theory and its development."
At thirty-five handwritten pages, however, the article Fundamentals and Methods of the Theory of Relativity proved too long for publication.
Nevertheless this article remains famous as Einstein described his discovery of the Equivalence Principle as the "happiest thought of my life".
 Grundgedanken und Methoden der Relativitätstheorie, in ihrer Entwicklung dargestellt