The concept of fact has a history. Over the past centuries, physicists have appropriated it in various ways. In this article, we compare Ernst Mach and Albert Einstein's interpretations of the concept. Mach, like most nineteenth-century physicists, contrasted fact and theory. He understood facts as real and complex combinations of natural events. Theories, in turn, only served to order and communicate facts efficiently. Einstein's concept of fact was incompatible with Mach's, since Einstein believed facts could be theoretical too, just as he ascribed mathematical theorizing a leading role in representing reality. For example, he used the concept of fact to refer to a generally valid result of experience. The differences we disclose between Mach and Einstein were symbolic for broader tensions in the German physics discipline. Furthermore, they underline the historically fluid character of the category of the fact, both within physics and beyond.This is amusing. I am not sure Mach and Einstein were really the trendsetters, but science popularizers today frequently use the term "fact" in a way that was not previously accepted.
Such a comparison assists the study of the modern notion of a "scientific fact," and how and why it should be distinguished from an "alternative fact." This is because Mach and Einstein's concepts of fact were constitutional for later and current notions, also outside of the physics discipline. Mach's fact-oriented empiricism was a primary source of inspiration for logical positivism and conventionalism, which in turn became hugely influential in shaping twentieth-century philosophical debates about realism, the relation between theory and experiment, and the role and status of scientific facts.15 Einstein's physics and philosophy, in particular his theory of relativity and his critique of quantum mechanics, also became an essential point of reference in such debates. What is more, Einstein actively contributed to epistemological discussions himself.16This paper has good historical info on the shift in thinking.
Half a century ago, Gerald Holton touched upon the main issue addressed by this paper. In 1968, Holton claimed that there was a "divergence between the conception of `fact' as understood by Einstein and `fact' as understood by a true Machist."17 According to Holton, this divergence related to the status of laws, concepts, and principles, which Mach, unlike Einstein, systematically distinguished from facts.