The only source of knowledge is experience.

This line is very widely credited to Einstein, almost always in this exact compressed form, and a serious reference work — The Yale Book of Quotations — points to his 1931 collection Cosmic Religion: With Other Opinions and Aphorisms as the likely origin. We could not, however, confirm the exact wording, page, or original context within that primary volume, and a full-text check of several Einstein titles turned up no verbatim match, which given known coverage gaps is inconclusive rather than negative. Our review of Ideas and Opinions, "Physics and Reality," and the Schilpp-edited Albert Einstein: Philosopher-Scientist found Einstein discussing the roles of experience, sense-perception, and reason at length, consistent with the sentiment, but not this precise aphorism. Notably, the same phrasing appears in older philosophical contexts — for example, a 1914 Queen's University examination paper and discussions of Kant and Hegel — which suggests it is a general epistemological maxim, but we found no single, named earlier author to whom this exact sentence can be credibly and specifically assigned, so it does not rise to a documented competing origin. Casual references to Hume or James reflect a shared empiricist tradition rather than a proven prior source, and no reputable scholar seriously contests Einstein's authorship in favor of a named alternative. With a plausible but unconfirmed lead in Cosmic Religion and no citable proof of a different originator, the honest position is that this is commonly credited to Einstein without firm primary documentation. We are marking this quotation as attributed.