To differentiate unsuccessful from successful experiments

I chased down this quote by Robert H. Goddard via The Goddard Papers Database . The original context of the letter, often omitted when using the quote, makes Goddard's words all the more delightful:

R. H. Goddard to F. T.*, Jeffersonville, Indiana
Roswell, February 5, 1942

It is not a simple matter to differentiate unsuccessful from successful experiments, since most work that is finally successful is the result of a series of unsuccessful tests in which difficulties are gradually eliminated...

*This was in response to a young experimenter who complained that many of his tests had been unsuccessful

SOURCE: Goddard, Robert H. The Papers of Robert H. Goddard. Edited by Esther C. Goddard and G. Edward Pendray. Vol. 3, 1941–1945: JATO and Variable Thrust for the Military. New York: McGraw-Hill, 1970, page 1453.