6 3 6 D O C U M E N T 3 9 9 K Y O T O L E C T U R E Published in Kaizo, Vol. 5, No. 2 (1923): 2–7. The original version was reprinted with a few minor revisions in Ishiwara 1923. The latter version was reprinted with further revisions in Ishiwara, H. 1971. Significant differences between the original publication and the reprinted version of 1923 are pointed out in the annotation. For a discussion of the differences between the first reprint and the sec- ond reprint of 1971, see Abiko 2000, pp. 3–4. TRANSLATION[1] It is by no means easy to give an account of how I arrived at the theory of relativity.[2] That is because it involves various hidden complex factors which sti- mulate one’s thinking and influence it in varying degree. I will not mention these factors one by one. Also I will not list the papers I have written. I will only briefly summarize the key points of the main strand in the development of my thinking. The first time I entertained the idea of the principle of relativity was some seven- teen years ago.[3] From where it came, I cannot exactly tell. I am certain, however, that it had to do with problems related to the optics of moving bodies. Light travels through the ocean of the ether, and so does the Earth. From the Earth’s perspective, the ether is flowing against the Earth. And yet I could never find proof of the ether’s flow in any of the physics publications. This made me want to find any way pos- sible to prove the ether’s flow against the Earth, due to the Earth’s motion. When I began pondering this problem, I did not doubt at all the existence of the ether or the motion of the Earth. Thus I predicted that if light from some source were appro- priately reflected off a mirror, it should have a different energy depending on whe- ther it moves in the direction of the Earth’s movement, or in the opposite direction. Using two thermoelectric piles, I tried to verify this by measuring the difference in the amount of heat generated in each. This idea was the same as in Michelson’s ex- periment, but my understanding of his experiment was not yet clear at the time.[4] I was familiar with the strange results of Michelson’s experiment while I was still a student pondering these problems, and instinctively realized that, if we ac- cepted his result as a fact, it would be wrong to think of the motion of the Earth with respect to the ether.[5] This insight actually provided the first route that led me to what we now call the principle of special relativity. I have since come to believe that, although the Earth revolves around the Sun, its motion cannot be ascertained through experiments using light. It was just around that time that I had a chance to read Lorentz’s monograph of 1895.[6] Lorentz discussed and managed to completely solve electrodynamics to first order approximation, i.e., neglecting quantities of the second order and higher of the ratio of the velocity of a moving body to the velocity of light. I also started to work on the problem of Fizeau’s experiment and tried to account for it on the