Never doubt that a small group of thoughtful, committed citizens can change the world: indeed, it’s the only thing that ever has

 --- attrib. to Margaret Mead. See QuoteInvestigator discussion, which concludes: "Donald Keys appears to be the crucial initial propagator of the quotation although it remains unclear how he learned about the statement. The precise phrasing and the ascription to Margaret Mead hinge on his veracity. There is no substantive support for competing ascriptions, and QI would tentatively assign the saying to Mead."