From his "Autobiographische Skizze" (18 April 1955), original German version here http://philoscience.unibe.ch/documents/kursarchiv/WS99/Skizze.pdf. Translation from Subtle is the Lord: The Science and the Life of Albert Einstein by Abraham Pais (1982), p. 131 http://books.google.com/books?id=U2mO4nUunuwC&lpg=PP1&pg=PA131#v=onepage&q&f=false. Pais notes that when he said "during that year", he was referring to some time between October 1895 and early fall 1896.
Variant: "Innovation is not the product of logical thought, even though the final product is tied to a logical structure."
Original German version: Während dieses Jahres in Aarau kam mir die Frage: Wenn man einer Lichtwelle mit Lichtgeschwindigkeit nachläuft, so würde man ein zeitunabhängiges Wellenfeld vor sich haben. So etwas scheint es aber doch nicht zu geben! Dies war das erste kindliche Gedanken-Experiment, das mit der speziellen Relativitätstheorie zu tun hat. Das Erfinden ist kein Werk des logischen Denkens, wenn auch das Endprodukt an die logische Gestalt gebunden ist. ("Autobiographische Skizze", p. 10)
1950s
Context: During that year in Aarau the question came to me: If one runs after a light wave with [a velocity equal to the] light velocity, then one would encounter a time-independent wavefield. However, something like that does not seem to exist! This was the first juvenile thought experiment which has to do with the special theory of relativity. Invention is not the product of logical thought, even though the final product is tied to a logical structure.
