“Hindu maxim that theory, speculations, [and] dogma change from time to time as the facts become better understood.”

Source: Ring (1994), Chapter 30 (p. 852)

Part I, Essay 6: Of The Independency of Parliament; first line often paraphrased as "It is a just political maxim, that every man must be supposed a knave."
Essays, Moral, Political, and Literary (1741-2; 1748)
Context: It is, therefore, a just political maxim, that every man must be supposed a knave: Though at the same time, it appears somewhat strange, that a maxim should be true in politics, which is false in fact. But to satisfy us on this head, we may consider, that men are generally more honest in their private than in their public capacity, and will go greater lengths to serve a party, than when their own private interest is alone concerned. Honour is a great check upon mankind: But where a considerable body of men act together, this check is, in a great measure, removed; since a man is sure to be approved of by his own party, for what promotes the common interest; and he soon learns to despise the clamours of adversaries.

Source: Times Go by Turns, Line 5; p. 47.

The earliest published attribution of this quote to Einstein found on Google Books is the 1991 book The Art of Computer Systems Performance Analysis by Raj Jain (p. 507), but no source to Einstein's original writings is given and the quote itself is older; for example New Guard: Volume 5, Issue 3 from 1961 says on p. 312 http://books.google.com/books?id=5BbZAAAAMAAJ&q=%22fit+the+theory%22#search_anchor "Someone once said that if the facts do not fit the theory, then the facts must be changed", while Product engineering: Volume 29, Issues 9-12 from 1958 gives the slight variant on p. 9 "There is an age-old adage, 'If the facts don't fit the theory, change the theory.' But too often it's easier to keep the theory and change the facts." These quotes are themselves probably variants of an even earlier saying which used the phrasing "so much the worse for the facts", many examples of which can be seen in this search http://www.google.com/search?tbo=p&tbm=bks&q=facts+fit+%22so+much+the+worse+for+the+facts%22&tbs=,cdr:1,cd_max:Dec%2031_2%201950&num=10; for example, the 1851 American Whig Review, Volumes 13-14 says on p. 488 http://books.google.com/books?id=910CAAAAIAAJ&pg=PA488#v=onepage&q&f=false "However, Mr. Newhall may possibly have been of that casuist's opinion, who, when told that the facts of the matter did not bear out his hypothesis, said 'So much the worse for the facts.'" The German idealist philosopher Johann Gottlieb Fichte circa 1800 did say "If theory conflicts with the facts, so much the worse for the facts." The Hungarian Marxist Georg Lukacs in his "Tactics and Ethics" (1923) echoed the same quotation.
Misattributed

Source: Faith vs. Fact (2015), p. 71

Der kategorische Imperativ, der überhaupt nur aussagt, was Verbindlichkeit sei, ist: handle nach einer Maxime, welche zugleich als ein allgemeines Gesetz gelten kann.
Source: Metaphysics of Morals (1797), Ch. 11

Interview with Euronews' Claudio Rocco in 2011; as quoted in " Science says 'ciao' to Italy's Margherita Hack: the 'lady of the stars'", euronews.com (1 July 2013) https://www.euronews.com/2013/07/01/science-says-ciao-to-italy-s-margherita-hack-the-lady-of-the-stars.