Explore Einstein's initial concept of general relativity as a variable speed of light theory in this 27-minute talk from the DPG Meeting in Berlin 2015. Delve into the historical aspects of Einstein's ingenious attempt between 1907-1911, examining an underestimated yet equivalent form of general relativity that allows for a Machian interpretation of gravity. Investigate topics such as the influence of gravitational fields, differential equations, and the dramatic fight for priority. Learn about the fatal error that led to this theory being overlooked, and discover connections between Einstein's work and that of other prominent physicists like Sir Arthur Eddington and Dirac. Gain insights into the Marx Principle and how it relates to Einstein's theories, ultimately understanding why this concept is considered one of the best ideas of the 20th century.