D O C U M E N T 3 9 9 K Y O T O L E C T U R E 6 3 7 assumption that the equations for the electron established by Lorentz also hold when the coordinate system of the vacuum is replaced by that of a moving body. At any rate, I believed at the time that the equations of Maxwell-Lorentz electrodyna- mics were secure and represented the true state of affairs. The circumstance, mo- reover, that these equations also hold in a moving coordinate system gives us a proposition called the constancy of the velocity of light. This constancy of the ve- locity of light, however, is incompatible with the law of the addition of velocities known from mechanics. Why do these two things contradict one another? I felt that I had come upon an extraordinary difficulty here. I spent almost a year fruitlessly thinking about it, ex- pecting that I would have to modify Lorentz’s ideas somehow. And I could not but think that this was a riddle that was not going to be solved easily. By chance, a friend of mine living in Bern (Switzerland) helped me.[7] It was a beautiful day. I visited him and I said to him something like: “I am struggling with a problem these days that I cannot solve no matter what I try. Today I bring this battle of mine to you.” I had various discussions with him. Through them it sudden- ly dawned on me. The very next day I visited him again and told him without further ado: “Thank you. I have already solved my problem completely.” My solution actually had to do with the concept of time. The point is that time cannot be defined absolutely, but that there is an inseparable connection between time and signal velocity. Using this idea, I could now for the first time completely resolve the extraordinary difficulty I had had before. After I had this idea, the special theory of relativity was completed in five weeks. I had no doubt that the theory was also very natural from a philosophical point of view. I also realized that it fitted nicely with Mach’s viewpoint. Although the spe- cial theory was, of course, not directly connected with Mach’s viewpoint, as were the problems later resolved by the general theory of relativity, one can say that there was an indirect connection with Mach’s analysis of various scientific concepts. Thus the special theory of relativity was born. The first thought leading to the general theory of relativity occurred to me two years later, in 1907, and it did in a memorable setting. I was already dissatisfied with the fact that the relativity of motion is restricted to motion with constant relative velocity and does not apply to arbitrary motion. I had always wondered privately whether this restriction could somehow be removed. In 1907, while trying, at the request of Mr. Stark, to summarize the results of the special theory of relativity for the Jahrbuch der Radioaktivität und Elektronik of which he was the editor,[8] I realized that, while all other laws of nature could be discussed in terms of the special theory of relativity, the theory could not be applied