2. The scientific reception of relativity

Early-20th-century American physics has long had a reputation for its empiricist orientation. Unlike many of their European colleagues, U.S. physicists typically presupposed a strictly empiricist philosophy of science, demanding a tighter connection between theory and observation than was usual at the time. Daniel Kevles writes about the community’s “arid form of empiricism” (Reference Kevles1979, 37), and Stanley Goldberg argues that physicists almost exclusively relied on empirical arguments in deciding between theories. The idea that theory choice depends on experimental evidence and theoretical virtues (e.g., simplicity or generality), widely accepted in Europe, was considered heresy in the United States (Goldberg Reference Goldberg1988, 79). U.S. physicists, Goldberg concludes, were often skeptical about abstract theorizing and exhibited a “general eschewal of metaphysics, which was identified with European culture” (Reference Goldberg1984, 267).

This empiricist approach is particularly evident in the community’s first responses to special and general relativity. In recent decades, historians have reconstructed the theory’s reception in a large number of countries, including Germany, England, France, China, Russia, Japan, Italy, Spain, and Belgium.Footnote ² This growing body of work reveals “the salience of national inflections” and shows that the reception of relativity was often colored by local scientific cultures (Glick Reference Glick1987, vii). In the decade after Einstein published his 1905 papers on the subject, special relativity was heavily debated in Germany but ignored in France, and the British were aware of it but largely stuck to the ether theory. In the United States, the responses were mixed, but both proponents and critics generally appealed to empiricist arguments in their writings. Whereas aesthetic-mathematical considerations played a major role in the responses of European scientists—even in England—American physicists generally ignored the question of whether the theory is mathematically elegant or contributes to a more unified physical theory.Footnote ³

The first American response to special relativity—Lewis and Tolman’s “The Principle of Relativity, and Non-Newtonian Mechanics”—was published in 1909. The two Massachusetts Institute of Technology (MIT) scientists discussed a number of recent experiments and analyzed to what degree the results supported special relativity. Likely inspired by A. A. Michelson’s 1908 Nobel Prize—the first to be awarded to a U.S. scientist—Lewis and Tolman focused mostly on the implications of the former’s ether experiments. Although the, at the time, diverse responses to these experiments show that their results can be variously interpreted, Lewis and Tolman claimed that the body of evidence left only one satisfactory explanation, namely, Lorentz’s conclusion that all moving bodies contract in the line of their motion (Reference Lewis and Tolman1909, 711–12). Einstein, the two acknowledged, was going a bit “beyond existing facts” in rejecting absolute motion altogether. But they were reasonably confident about the possibility of “further verification” because Einstein had deduced additional empirical consequences from his hypothesis (Lewis and Tolman Reference Lewis and Tolman1909, 712, 718). As such, Lewis and Tolman concluded, the principle of relativity appeared to be “established on a pretty firm basis of experimental fact” (712–13). The first English-language book on relativity, by the U.S. mathematician R. D. Carmichael, also relied on empiricist arguments. Although many mathematicians evaluated the theory’s formal properties, Carmichael exclusively focused on its empirical support. Like Lewis and Tolman, his conclusions were cautiously optimistic. Carmichael concluded that “there is no experimental evidence which is undoubtedly opposed” to the theory, although there may be indirect evidence in its favor (Reference Carmichael1912, 18–19, 63–65).Footnote ⁴

Opponents of relativity theory appealed to empiricist considerations, too. W. F. Magie, one of the founding members of the American Physical Society, objected to what he deemed to be a metaphysical theory, arguing that “Michelson-Morley” only supported the conclusion that there is no way to determine the relative motion of Earth and the ether when the observer and the source of light are moving along with our planet (Reference Magie1912, 288). To abandon absolute motion altogether would be to draw an empirically unwarranted conclusion. His colleague L. T. More, a professor of physics at the University of Cincinnati, was equally worried that Einstein’s principle obliterated “the boundary between science and metaphysics” and argued that it transcended the discussion of postulates “determined by experience” (Reference More1911, 196). Interestingly, the two critics disagreed about where to draw the distinction between physics and metaphysics. Magie was a staunch defender of the ether theory and believed it to be the only empirically plausible explanation of the transmission of light. More objected to any speculation about the nature of the cosmos and submitted that both “atoms and ethers … are metaphysical creations” (Reference More1910, 815). A true scientist, More argued, is exclusively concerned with the formulation of laws “deduced mathematically from experimental data” (Reference More1909, 876).

Empiricism is not just a view about epistemic justification. Typically, empiricists also believe that theoretical terms should have the appropriate semantic connection to observational concepts. Indeed, some of the most sophisticated treatises of the period also emphasized this conceptual side of empiricism. In doing so, they often followed Peirce, who had argued that the meaning of a hypothesis is determined by its experimental effects, or J. B. Stallo, a German-born philosopher of science who had published The Concepts and Theories of Modern Physics in Reference Stallo1882. Stallo defended a quasi-positivist perspective and warned against the reification of basic physical concepts. He primarily used his approach to criticize what he deemed to be the metaphysical assumptions of Newtonian physics, rejecting absolute space, absolute time, and absolute motion. In eliminating from science “its latent metaphysical elements,” he hoped to contribute to the scientific endeavor to gain “a sure foothold on solid empirical ground, where the real data of experience may be reduced without ontological prepossessions” (Reference Stallo1882, 8).Footnote ⁵

Both the theory of special relativity and that of general relativity stimulated American physicists to further reflect on the conceptual foundations of their discipline, not in the least because Einstein himself appeared to give a positivist spin to his discovery. In writing about the “profound influence” of Mach and in arguing that a “concept does not exist for the physicist until he has the possibility of discovering whether or not it is fulfilled in an actual case” (Reference Einstein1917, 22), Einstein seemed to presuppose a positivist perspective, inspiring U.S. scientists to adopt a similar approach.Footnote ⁶ In 1927, the Harvard physicist P. W. Bridgman published The Logic of Modern Physics, a book that applied the lessons of, among others, Stallo and Mach to the recent revolutions in physics (Reference Bridgman1927, v–vi). His solution was to adopt a strictly empiricist, or operationist, attitude toward the concepts of physics, exemplified in his mantra that we “mean by any concept nothing more than a set of operations” (Reference Bridgman1927, ix–x). Although Bridgman was critical of general relativity, he believed he was criticizing Einstein on Einsteinian grounds, using the latter’s perspective on “what the concepts useful in physics are and should be” (Reference Bridgman1927, 4). In equating the meaning of statements about simultaneity with the concrete operations we use to determine whether two events occur simultaneously, Einstein had developed an operational analysis of time in his 1905 papers. Moreover, he had repeated the point in his more popular Relativity: The Special and General Theory (Einstein Reference Einstein1917).

The crucial difference between Bridgman’s perspective and traditional approaches is the way concepts are conceived. Classical physicists often defined concepts in terms of properties. Newton, for example, defined absolute time as that what “of itself, and from its own nature flows equably without regard to anything external” (Bridgman Reference Bridgman1927, 4). The danger of this approach is that we might discover that there is nothing in nature that has these properties, so we are constantly confronted with scientific revolutions like the one sparked by Einstein. Bridgman instead proposed to define concepts in terms of operations. Applied to Newton’s concept of absolute time, this means that we do not understand its meaning “unless we can tell how to determine the absolute time of any concrete event”; once we see that the operations by which we measure time are relative, as Einstein demonstrated, we have to conclude that the concept is operationally “meaningless” (Bridgman Reference Bridgman1927, 5). In order to prevent similar revolutions in the future, we have to subject all concepts of physics to an operational analysis.

3. American philosophy at the turn of the century

Early discussions of relativity were mostly confined to a small circle of physicists, astronomers, and mathematicians. This changed in November 1919, when the Royal Society announced that Einstein’s predictions about the bending of starlight had been confirmed by Eddington’s solar eclipse expedition. In the years after the announcement, American media published hundreds of articles trying to explain the theory. Einstein was described as the “destroyer of space and time” and became a national celebrity when he visited the country in 1921 (Missner Reference Missner1985, 271–73). Given this widespread attention for topics that had traditionally been the domain of philosophy—space and time—it should not be a surprise that philosophers quickly started to write about relativity, too.

Historians often divide early-20th-century American philosophy into three distinct but partially overlapping schools: idealism, realism, and pragmatism (Kuklick Reference Kuklick2001; Campbell Reference Campbell2006, ch. 3). The most sizable of the three, the idealist movement, was skeptical about the empiricist approach that dominated the sciences. Most idealists believed that experimental findings can, at best, deliver a partial understanding of reality. Two hundred years of modern epistemology had shown that empiricism leads to skepticism because there is no way to determine whether our ideas correspond to an independently existing material world. Instead of blindly relying on science, we should accept that reality is mind dependent and that there are moral and spiritual dimensions to experience, too. The idealists held that it is the philosophers’ job to ground physical, moral, and religious truths and unify these domains into a coherent system. Only philosophy, idealists believed, can “investigate the grounds … of the whole body of truth with a view to its unity and meaning as a whole” (Ormond Reference Ormond1906, 3).

It is no coincidence that idealism dominated philosophy at the turn of the century because the development of philosophy as a distinct academic discipline was partly a response to the increasing influence of science in American academia. The establishment of dozens of new laboratories and polytechnics, the rise of experimental psychology, and the popularity of philosophically minded naturalists such as Ernst Haeckel and Herbert Spencer contributed to the feeling that philosophy was in danger of being swallowed by the sciences (Campbell Reference Campbell2006; Wilson Reference Wilson1990). In response to this threat, idealists helped found the first professional journals (e.g., Philosophical Review in 1892) and organizations (e.g., American Philosophical Association [APA] in 1902) to establish philosophy as an independent discipline. The first president of the APA, the idealist J. E. Creighton, argued that philosophy had to protect itself against scientists who “wholly unschooled in the subject … feel themselves competent … to write philosophical books” (Reference Creighton1902, 232).

The idealist movement began to be challenged in the first decade of the 20th century. In England, the revolt was led by G. E. Moore and Bertrand Russell, two Cambridge graduates who objected to the views of their idealist colleagues and sought to replace them with a variant of realism. Something similar happened in the United States. Two recent graduates from “the other Cambridge”—W. P. Montague and R. B. Perry—objected to the views of Josiah Royce, America’s best-known idealist (Montague Reference Montague1902; Perry Reference Perry1902). Inspired by Russell’s work, they argued for a more scientific approach to philosophy. Whereas their idealist predecessors distinguished between scientific findings and philosophical synthesis, these “new realists” viewed themselves as part of an “era of united and complimentary endeavor” (Holt et al. Reference Holt, Marvin, William Pepperell Montague, Pitkin and Gleason Spaulding1912, 21). They promoted the use of mathematical logic, analytic methods, and a piecemeal approach, dealing with “one problem at a time” instead of attempting to “answer all questions together” (Holt et al., 21–26). The “most notable feature of a realistic philosophy,” the realists believed, “is the emancipation of metaphysics from epistemology” (Holt et al., 32). Whereas the idealists had put epistemology center stage, using the theory of knowledge to draw conclusions about the nature of reality, the realists turned this relation on its head, arguing that the knowledge relation is just one of many relations between independently existing objects.

The second, and nowadays best-known, alternative to idealism was developed by the pragmatists. Building on the work of, among others, Peirce, James, and Dewey, pragmatism became an influential yet diverse philosophical movement that was more closely tied to the empiricist tradition in the sciences. William James had been a crucial figure in the development of experimental psychology, and Peirce’s aforementioned criterion of meaning implied that two hypotheses have the same content if they have the same observational consequences. James first invoked C. S. Peirce’s pragmatic test in an 1898 paper and used it to argue that many speculative debates about the nature of reality are pointless. Dewey had started out as a Hegelian but came to replace his idealist approach with a naturalized perspective on man, mind, and morality.

4. The philosophical reception of relativity

The 1919 Royal Society announcement gave new impetus to the debate between idealists, realists, and pragmatists. In the decade after the news about Eddington’s expedition, American philosophy journals published dozens of papers and reviews on relativity, discussing its foundations and philosophical implications.Footnote ⁷ Table 1 lists most of the papers published in three prominent American philosophy journals—the Journal of Philosophy, Philosophical Review, and The Monist—between 1921 and 1930 and shows that these periodicals published a host of articles on relativity theory.Footnote ⁸ Table 2 lists most of the reviews of books on relativity theory in two of these journalsFootnote ⁹ and reveals that the philosophical community also kept a close eye on foreign publications on the subject, even if they were written by physicists.

A closer study of the papers listed in Table 1 shows that the theory was discussed by philosophers from a variety of schools. Idealists, realists, and pragmatists, but also philosophers representing smaller movements such as Bergsonism and phenomenology, responded to relativity, and many of them were convinced that Einstein’s principle supported the perspective they had been developing themselves. H. R. Smart, who regularly reviewed books on relativity (Table 2), said that many philosophers viewed relativity as a “welcome vindication of their particular philosophical doctrines” (Reference Smart1925, 511), and Russell wrote that “there has been a tendency, not uncommon in the case of a new scientific theory, for every philosopher to interpret the work of Einstein in accordance with his own metaphysical system” (Reference Russell1926, 331).

Many idealists felt vindicated by relativity because they took Einstein to have shown that there is no mind-independent order of temporal relations. Realists had argued that space and time have an objective existence, but Einstein’s theory, these idealists held, revealed this to be a mistake. Whereas the aforementioned Montague had characterized reality as a distribution of qualities over an independently existing four-dimensional manifold of spatial and temporal positions (Reference Montague, Edwin, Marvin, Montague, Perry, Pitkin and Spaulding1912, sec. 1–2), these idealists believed Einstein to have shown that even a basic property like length belongs not to an independently existing object but exists “as a relation of observer-and-observed” (Smith Reference Smith1921, 505). In drawing these conclusions, they followed the British philosopher R. B. Haldane, who argued that “if the principle of relativity is well-founded the very basis of the New Realism seems to disappear into vapour” (Haldane Reference Haldane1921, 273). And they were likely inspired by the London-based philosopher H. W. Carr, who held that Einstein’s theory shows that there is no “concrete four-dimensional space-time” that serves as the substratum of our activities but that there are only the “perception-actions of infinite individual creative centres in mutual relation” (Carr Reference Carr1920, 162). In 1922, Carr even organized a debate at the Aristotelian Society on the thesis that the “principle of relativity … is in complete accord with the neo-idealist doctrine in philosophy, and in complete disaccord with the fundamental standpoint of every form of neo-realism” (Carr et al. Reference Carr, Percy Nunn, Whitehead and Wrinch1922, 123).Footnote ¹⁰

In the United States, this reading was defended by a number of philosophers, including mathematician-philosopher William B. Smith. In a paper titled “Relativity and Its Philosophic Implications,” Smith developed the thesis that relativity was a further step into the direction of a view in which “objects … are not discoveries but the creations of psychic activity” (Reference Smith1921, 505). The Tulane professor was working on a book titled Mind: The Maker and was convinced that Einstein’s theory fitted “completely and perfectly … with the general world-view that I have long cherished and am gradually shaping into expression” (Reference Smith1921, 509). Another example is the Russian-American philosopher Wolf Gordin, who argued that Einstein had “disproved” those who would “banish philosophy from the realm of reality”; Gordin believed that Einstein had set in motion an “an unsurmized renaissance of philosophy, mathematics, logic, epistemology, and metaphysics” that combined non-Euclidean geometry with Cantor’s work on infinity and “Hegel’s dialectics” (Reference Gordin1926, 518).

Many realists, on the other hand, were critical of relativity, and some of them even tried to dismiss the theory.Footnote ¹¹ Montague, for example, published an analysis of special relativity and concluded that Einstein’s ideas are internally inconsistent. One of his central arguments was a version of the twin paradox, in which one of two twin brothers travels back and forth into space and discovers, upon return, that he has aged less than his stay-at-home brother. Applying the relativity of motion, such that the stay-at-home brother could also be viewed as the one who has been traveling back and forth in the opposite direction, Montague derived the paradoxical conclusion that each twin is younger than his brother (Reference Montague1924, 156). In order to resolve the paradox, Montague proposed an alternative to special relativity built on the assumption that the speed of light is, pace Einstein, affected by the velocity of its source. In fact, Montague even sketched an experiment designed to test his alternative and called upon the readers of Philosophical Review to help and fund it: “The cost of the experiment might run to $20,000. … Perhaps some of you will be willing to pray that there be sent to me a kind-hearted rich man who will take a sporting chance and put up the necessary funds” (Reference Montague1924, 162).

Montague was not the only philosopher to make use of the twin paradox to dismiss special relativity. A few years later, Arthur Lovejoy published an article sketching a similar paradox (Reference Lovejoy1931). Lovejoy, who had a stake in the debate because he had long defended a position that has been dubbed “temporal realism” (Kurz Reference Kurz1966, 354), believed it was simply inconsistent to dismiss the assumption “that there is a single universal order of temporal relations … in which every event can be unequivocally assigned” (Lovejoy Reference Lovejoy1930, 617). Lovejoy’s most important objection to relativity, however, concerned Einstein’s theory of meaning. Turning Bridgman’s modus ponens into a modus tollens, Lovejoy accepted the latter’s diagnosis that Einstein presupposed a “radically experimental theory of meaning” but used it to reject special relativity. It is simply “preposterous,” Lovejoy argued, to suppose that “no term can ever signify anything more than what is actually given in the verifying experience”:

Mocking Einstein’s criterion, Lovejoy argued that it implied that if a bedridden patient observes two men, one outside in the rain and another entering her room with wet clothing, her inference that rain had fallen upon both could not have the same meaning in the two cases because it had been verified in a different way (Reference Lovejoy1930, 628). Instead, Lovejoy proposed an alternative theory of meaning in which an experimental finding is “the sign or circumstantial evidence of something else,” not the “meaning” of the term (Reference Lovejoy1930, 620).

Not everyone accepted Lovejoy’s argument. A substantial group of philosophers embraced Bridgman’s conclusions and interpreted them as confirming a broadly pragmatist orientation. They felt emboldened by the Logic of Modern Physics because they read it as offering an essentially Peircean perspective on scientific concepts. J. S. Bixler argued that Bridgman’s “new physics” confirmed the “pragmatic theory that knowledge is directed toward the consequences of experimental operations” (Reference Bixler1930, 214). And Ernest Nagel saw Bridgman’s perspective as a new version of the approach Peirce had developed 50 years before:

W. E. Van de Walle even suggested that Bridgman’s book could have been titled “The Evidence from Physics for Pragmatism” (Reference Van de Walle1928, 286). Einstein had ignited an intellectual firestorm, and one of the country’s most prominent physicists advanced a perspective that sounded very much like the view pragmatists had been defending for decades.

5. Scientific philosophy

Many of the aforementioned papers are relatively shallow when compared to some of the work that was published in, for instance, Germany and England. It is unlikely that philosophers such as Smith, Gordin, Montague, Lovejoy, Bixler, and van de Walle fully understood Einstein’s theory. Montague’s version of the twin paradox had already been resolved when he published his paper.Footnote ¹³ And Lovejoy’s reading of Einstein’s theory of meaning was quickly rejected by Evander McGilvary, who showed that the Swiss-German professor had never claimed that concepts ought to be defined in terms of the operations we use to test them. Einstein, McGilvary argued, defended a subtler criterion in which concepts are only indirectly tied to operations. A circle, for example, is not defined in terms of the method we use to determine whether a particular shape qualifies as a circle. Conversely, we use the definition of a circle—a figure consisting of points equidistant from a given point in a two-dimensional plane—to find a method for “how to go about finding out whether a figure is a circle” (McGilvary Reference McGilvary1931, 427).

Still, several American philosophers made lasting contributions in the wake of Einstein’s discoveries. One important example is Theodore de Laguna, whose work on geometry helped found the field of mereotopology (Reference De Laguna1922a, Reference De Laguna1922b, Reference De Laguna1922c). De Laguna, a Bryn Mawr professor, proposed to define standard spatial concepts such as “point” and “coordinate position” in terms of region-based concepts such as “solid” and “connection,” instead of the other way around, and is today still considered “a forerunner” in the area of qualitative topological reasoning (Varzi Reference Varzi, Aiello, Pratt-Hartmann and van Benthem2007, 979). Not only did it influence Whitehead’s work on the relation of extensive connection (Whitehead Reference Whitehead1929, 287), but present-day mathematicians still view him as one of the first scholars to develop a region-based geometry (Pratt-Hartmann Reference Pratt-Hartman, Aiello, Pratt-Hartmann and van Benthem2007, 91). Another set of valuable contributions came from Yale, where an interdisciplinary group of philosophers and physicists—Filmer Northrop, Henry Margenau, and Fred Hoskyn—regularly contributed to debates about the methodological implications of relativity (see Table 1). All three were critics of Bridgman’s view and aimed to develop a theory of meaning that allows theoretical constructs.Footnote ¹⁴ Whereas Bridgman held that we employ different concepts of length if we use different operations to measure length in different domains, Margenau believed that such a criterion dissolves reality into an “unmanageable variety of discrete concepts without logical coherence”:

Instead, Margenau and Northrop introduced a separate category of concepts—“concepts by postulation”—and argued that modern physical theories, including Einstein’s mechanics, require such notions. Although concepts by postulation cannot be operationally defined, theories involving such concepts are testable because one can derive consequences from them that can be directly verified (Northrop Reference Northrop1939, 434–35).Footnote ¹⁵

Although only a few of the articles listed in Table 1 have withstood the test of time, it would be a mistake to conclude that this literature has been rightly ignored by historians. On the contrary, these philosophical discussions about relativity are significant because they contributed to the development of a uniquely American branch of philosophy of science. Whereas the first decades of the century were marked by philosophical disputes between idealists, realists, and pragmatists, participants in the debate about relativity contributed to the development of a more scientifically oriented philosophy. Even some idealists, who had traditionally been suspicious of overly scientistic approaches (see sec. 3), now explicitly recognized the “the dependence of philosophy upon the findings” of the special sciences (Northrop Reference Northrop1925, 6). Unlike the situation in Italy, where “neo-idealists just dismissed the question of the philosophical consequences of relativity” because the “idea of an idealist science” would be “a contradiction in terms” (Reeves Reference Reeves and Thomas1987, 206–8; Sanchez-Ron Reference Sanchez-Ron, Lehner, Renn and Schemmel2012), several American idealists helped promote the idea that philosophy should become more scientific. And although some opponents of idealism were skeptical about the value of metaphysical speculation, most of them could live with a speculative movement that had “its feet on the ground, however much its head may swim” (Costello Reference Costello1931, 245). Einstein’s theory, in other words, stimulated philosophers to develop more scientifically informed perspectives. Perry, the aforementioned realist, even wrote a paper in which he signaled that the scholastic disputes that had characterized U.S. philosophy before World War I (see sec. 3) had made a place for an “era of philosophical peace” because science had given everyone “something new to think about” (Perry Reference Perry1928, 311–12).Footnote ¹⁶

Conversely, physicists and mathematicians also became increasingly interested in the philosophical foundations of their disciplines. Bridgman and Margenau were certainly not the only scientists to do work in the philosophy of physics. The aforementioned Carmichael wrote a paper on the “philosophical implications” of relativity (Carmichael Reference Carmichael and Bryan1927) and published a textbook titled The Logic of Discovery (Carmichael Reference Carmichael1930). The mathematician G. D. Birkhoff developed an axiomatization of general relativity (much like Reichenbach had done in Berlin), adding a chapter on the theory’s “philosophical influence” (Birkhoff Reference Birkhoff1925). And the Columbia mathematician C. J. Keyser, one of the American postulate theorists, published a book titled Mathematical Philosophy in which he aimed to bring philosophers and mathematicians closer to one another (Keyser Reference Keyser1922).

Together, this growing community of scientists and philosophers started to develop a new field that was variously called “scientific philosophy” or “philosophy of science.”Footnote ¹⁷ In 1925, C. I. Lewis signaled the rise of a “new movement in philosophy” inspired by the “revolutionary advances in logic, in mathematical, and in physical theory” and noted that “the partitions between these subjects have become thin or disappeared” as they all developed “in the direction of greater comprehensiveness and increased rigor” (Reference Lewis1925, 410). Similar observations were made by Paul Schilpp, who recognized “a tendency in recent American philosophy which … may perhaps most adequately called and described as the philosophy of science” (Reference Schilpp1930, 276); by Frank Thilly, who noted the rise of “new movements” that “derive their inspiration from the methods and results of natural science … and seek … to avoid the metaphysical presuppositions of the older schools” (Reference Thilly1926, 522); and by Charles Morris, who recognized “many streams of activity” that contribute to “a wide convergence toward a unified philosophical science and scientific philosophy” (Reference Morris1935, 147–48; Verhaegh Reference VerhaeghForthcoming).

6. Logical empiricism

Although it is difficult to estimate the relative size of the community of philosophers and scientists involved in debates about the foundations of science, there is quite a lot of evidence that the American reception of logical empiricism was directly connected to the previously discussed debates. German and U.S. philosophers had worked in relative isolation since the First World War, but the philosophy of relativity functioned as a shared reference point when the Allied boycott on German scholarship was lifted in 1926. Whereas Reichenbach’s first two books on relativity, published during the years of the boycott, had been generally ignored, for example, his 1928 Philosophie der Raum-Zeit-Lehre was positively reviewed and regularly cited in the American literature (Langer Reference Langer1930; Northrop Reference Northrop1931). Susanne Langer, for example, praised Reichenbach’s approach to philosophy of science because it concerned “the philosophical reflection of a scientist, not the scientific speculation of a philosopher” like “Einstein, Whitehead, or Weyl” before him (Reference Langer1930, 611). Moritz Schlick, who had written one of the best-known philosophical works on relativity in the German-speaking world, was quickly invited to come and lecture in the United States. H. W. Stuart, chairman of Stanford’s philosophy department, had been reading Schlick’s exposition of Einstein’s theory and wrote that it would be his great pleasure to welcome him to California.Footnote ¹⁸ And Philipp Frank, Einstein’s successor in Prague, was invited to do a lecture tour in the United States, where he was greatly admired for his understanding of “modern physics and philosophy” and his competence “to treat these two fields jointly.”Footnote ¹⁹ Although the American reception of logical empiricism is commonly tied to its “philosophical” program, many of the first encounters between U.S. philosophers and logical empiricists were concerned with the philosophical implications of relativity.

The logical empiricists, in turn, were also interested in the perspectives on relativity that had been developed in the United States. Schlick read up on Bridgman’s work before he traveled to the United States and published a review of The Logic of Modern Physics in Die Naturwissenschaften. And Schlick’s student Herbert Feigl even acquired a Rockefeller fellowship to study with Bridgman at Harvard in the 1930–31 academic year. Carnap had already discovered the diverse Anglophone literature on relativity in 1923, when he had attended a congress of the American Mathematical Society in New York. In a letter to Reichenbach, Carnap described the growing “interest in … mathematical logic” and surveyed the Anglophone literature on relativity. His letter includes a list of English-language publications on Einstein’s theory (including, among others, Carmichael’s and Keyser’s books) and expresses his surprise about the amount of “valuable work that has been done and is important for us.”Footnote ²⁰

The philosophy of relativity, in sum, stimulated European and American philosophers to get acquainted with each other’s work, thereby paving the way for the relatively warm reception of logical empiricism in the United States.Footnote ²¹ Some American philosophers even traveled to Europe to visit the Berlin Group and/or the Vienna Circle. The Yale philosopher Northrop acquired a fellowship to visit Reichenbach and Einstein in Berlin, describing Reichenbach’s work as exactly “the kind of thing we need in philosophy.”Footnote ²² The New York philosopher Sidney Hook visited Reichenbach in Germany and described Philosophy der Raum-Zeit-Lehre as “the most lucid and comprehensive exposition of the philosophical implications of the theory of relativity” (Hook Reference Hook1930, 159). And the critical realist C. A. Strong invited Feigl to come to his Italian residence in Fiesole in order to help the American philosopher acquire “a better understanding of Einstein’s theory of relativity.” Strong “was working on a metaphysical theory of space and time and wanted to find out to what extent his views were compatible with those of Einstein” (Feigl Reference Feigl, Fleming and Bailyn1969, 68). Feigl, in turn, traveled to the United States to work with Bridgman and learn more about his operationist approach to the philosophy of physics and, in doing so, helped spread the views of the Vienna Circle to some of the philosophers (most notably, C. I. Lewis and W. V. Quine) who would come to play an important role in the further promotion of logical empiricism in the United States.Footnote ²³

A few years later, several members of this diverse community of European and American philosophers (idealists, pragmatists, operationists, realists, and logical empiricists) would become involved in the two boards of Philosophy of Science, which published its first issue in 1934, thereby contributing to the institutionalization of philosophy of science in North America. Indeed, the editorial and advisory boards of this new journal perfectly reflect the intellectually diverse community of philosophers of science working on relativity in this period. The team included—in addition to editor William Malisoff—Bridgman, Carnap, Feigl, Lovejoy, Margenau, Montague, Northrop, Reichenbach, Schlick, and Whitehead.