In a 1919 article for the Times of London, Einstein declared the relativity to be a ‘principle theory’, like thermodynamics, rather than a “constructive theory”, like the kinetic theory of gases.

As I have shown elsewhere, Einstein primarily used the relativity/thermodynamics analogy as a negative defensive argument to address the concerns of his opponents, particularly the abstract nature of the theory. In this brief note, I show, how, in his later years, Einstein began to use the relativity theory/thermodynamics comparison not merely as a post hoc justification but as a description of the path he had followed in developing special relativity. In particular, Einstein emphasized that he was motivated by his skepticism toward Maxwell’s equations. The Lorentz transformations are not simply a by-product of Maxwell’s equations or any specific dynamical law. Like the principles of thermodynamics, they were derived from well-established empirical facts. For this reason, they could be elevated to a constraint that all laws of nature must satisfy. Additional results are obtained by modifying the existing laws of nature valid in the limiting case so that they satisfy this constraint.