Hume and Einstein’s Special Theory of Relativity

Matias Slavov

University of Jyväskylä

In the 14th of December, 1915, Einstein wrote a letter to Schlick where he declared that it was “Mach, and, even more, Hume, whose Treatise of Human Nature I studied with passion and admiration shortly before discovering the [special] theory of relativity. Very possibly, I wouldn't have come to the solution without those philosophical studies.” Though much ink has been spilled about Mach's influence to Einstein's science, the relationship between Hume's philosophy and Einstein's philosophical analysis related to the special theory of relativity is far less known. What was it about Hume's Treatise, whose german translation Einstein read in the early 1900's, and which was discussed extensively in the “Olympia academy” reading group that Einstein participated in Bern that he found beneficial to the formulation of the special theory of relativity? The quote above does not tell what it was about Hume's philosophy that Einstein implemented to his science. So, what type of connection is there between the two? To answer this question, perhaps the most comprehensive and concise analysis is presented in John D. Norton's article “How Hume and Mach Helped Einstein Find Special Relativity” (2009). Norton's main thesis is that Einstein was most influenced by the way Hume saw concepts, or ideas, to be grounded in experience. If the concept of simultaneity is grounded in sensible experience, such as in a visual experience of immediate light flashes in two mirrors, it follows that there is no absolute simultaneity, since different inertial reference frames observe the timely order of two spatially distant events, the two light flashes, in different order. In addition to Norton’s observation, I contend that there are still at least two important intertwined points between Hume's and Einstein's views. First, they both support an empiricist theory of ideas or concepts. According to their views ideas and concepts are justified by and made meaningful by impressions or sensations. Second, they both insist that adequate ideas or concepts of space and time refer relatively to individual bodies. I shall argue that these two points are symmetrical in Hume and Einstein: the empiricist theory of ideas/concepts entails that the notions of space and time require observable physical objects; in turn, the ideas/concepts of physical objects can be justified and acquired by relevant sensuous impressions, such as vision and touch. As a consequence, their views are relationist and stand in opposition to Newton's understanding of space and time as being absolute. However, though I find Hume's and Einstein's views to overlap about the two points mentioned above, I will argue that Einstein's empiricist remarks about concepts referring to sensations instantiate a more mitigated empiricism than Hume's copy principle. Einstein frequently states that concepts are the “free creation of the human mind,” and that they are not deducible from sensations, or connected to them in any logically necessary way. This is not the case with Hume, since according to his copy principle impressions determine ideas—there is a complete isomorphy between the two—and our will regarding this relation is not free.