Back when I interviewed the (now late) philosopher Robert Nozick about his final book Invariances, I asked him about a phrase in the book that had puzzled me: He had referred to superstring theory as an interesting branch of metaphysics. He replied:

Well, I think that science, as it probes realms that aren’t just at the level of ordinary macroscopic objects that we look at, uncovers strange phenomena, unusual and surprising phenomena that we didn’t expect that leads the scientist to formulate radically new theories about what’s going on. And those are conceptually very interesting, and philosophers ought to be interested in other conceptual possibilities, especially when there’s some reason to think that they might be actualities and not just possibilities. The scientists often have more unfettered imaginations than current philosophers do. Relativity theory came as a complete surprise to philosophers, and so did quantum mechanics, and so did other things. So I think that scientific theories are of great philosophical interest, and that they stimulate the philosophical imagination and mind. And they tend to undercut some of the traditional categories that we take for granted.

I don’t think I fully appreciated what an insightful point this was until I began reading (OK, OK, listening to the audiobook of) Walter Isaacson’s wonderful biography Einstein: His Life and Universe. (I should admit here that my interest in Albert Einstein is not wholly unrelated to the fact that, in this inertial reference frame, he was born 100 years to the day before I was. But then, since motion is relative, there’s a sense in which I’m justified in thinking the world revolves around me…) Because I now realize there’s a very literal sense in which Einstein was really doing philosophy at least as much as he was doing science. Not just because his subject matter—the fundamental nature of time and space—obviously overlaps with the concerns of metaphysicians, but because the methods that led to his major discoveries look (to me, anyway) much more like what we associate with philosophical reasoning than with the scientific method of hypothesis construction and experimental testing, which he largely left to others.

Note, for instance that the most famous of Einstein’s four legendary Annus Mirabilis papers, “On the Electrodynamics of Moving Bodies,” is essentially a series of thought-experiments given mathematical expression; the paper cites no other contemporary scientific publications. Einstein himself would later emphasize that perhaps his most important intellectual influence as he was developing the theory of Special Relativity was not another physicist, but the Scottish philosopher David Hume, who had cautioned against imputing objective reality to abstractions that extended beyond experiential evidence.

Einstein was also, apparently, influenced by the early positivists, who insisted that the meaning of any concept, if it was to be coherent, had to be reducible to specific processes or observations. The key to Einstein’s great breakthrough was not any experimental data, but his willingness to pose a series of philosophical-sounding questions—what do we mean when we say that two events are “simultaneous”?—and to interpret concepts like “time” not as corresponding to some kind of nebulous entity, but in terms of the operations we perform when making and verifying statements about time. This is how he hit upon the insight that all our talk about “time” is ultimately talk about the relative motion of physical systems (like clocks). This at least arguably prefigures points of the sort Wittgenstein would later make in his Philosophical Investigations. As Nozick points out, then, Einstein was not just one of the greatest physicists who ever lived (and even the qualification there is doubtful) but also a far better and more imaginative philosopher than most philosophers.