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Source: "Mathematical retrospections," 2013, p. 142
As quoted in Faust in Copenhagen (2007) by Gino Segrè, p. 130.5, which cites The Historical Development of Quantum Theory (1982) by Jagdish Mehra and Helmut Rechenberg, vol 1 of 4, p. xxiv, and Inward Bound (1986) by Abraham Pais, p. 186
                                        
                                         Speech to Conservative Central Council (15 March 1975) http://www.margaretthatcher.org/document/102655 
Leader of the Opposition
                                    
Interview: "Leonardo DiCaprio: 'wealth and success don't make you happy'" http://www.telegraph.co.uk/film/the-revenant/leonardo-dicaprio-interview/ by Chloe Fox, The Telegraph (9 January 2016)
                                        
                                         In a video posting, announcing his candidacy for President of the United States (16 January 2007) http://www.barackobama.com/video/from_barack_transcript/ 
2007
                                    
Lecture on "Electrical Units of Measurement" (3 May 1883), published in Popular Lectures Vol. I, p. 73, as quoted in The Life of Lord Kelvin (1910) by Silvanus Phillips Thompson
                                        
                                        In Carl Seelig's Albert Einstein: A Documentary Biography (1956), Seelig reports that Einstein said this to James Franck,  p. 71 http://books.google.com/books?id=VCbPAAAAMAAJ&q=%22how+it+happened%22#search_anchor. 
I sometimes ask myself how did it come that I was the one to develop the theory of relativity. The reason, I think, is that a normal adult never stops to think about problems of space and time. But my intellectual development was retarded, as a result of which I began to wonder about space and time only when I had already grown up. Naturally, I could go deeper into the problem than a child with normal abilities. 
Variant translation which appears in Einstein: The Life and Times by Ronald W. Clark (1971),  p. 27 http://books.google.com/books?id=6IKVA0lY6MAC&lpg=PP1&pg=PA27#v=onepage&q&f=false 
Attributed in posthumous publications
                                    
 
                            
                        
                        
                         
                            
                        
                        
                         
                            
                        
                        
                         
                            
                        
                        
                        