Albert Einstein (1879–1955) is well known as the most prominentphysicist of the twentieth century. His contributions totwentieth-century philosophy of science, though of comparableimportance, are less well known. Einstein’s own philosophy ofscience is an original synthesis of elements drawn from sources asdiverse as neo-Kantianism, conventionalism, and logical empiricism,its distinctive feature being its novel blending of realism with aholist, underdeterminationist form of conventionalism. Of special noteis the manner in which Einstein’s philosophical thinking wasdriven by and contributed to the solution of problems firstencountered in his work in physics. Equally significant areEinstein’s relations with and influence on other prominenttwentieth-century philosophers of science, including Moritz Schlick,Hans Reichenbach, Ernst Cassirer, Philipp Frank, Henri Bergson,Émile Meyerson (see Russo-Krauss and Laino 2024).

• 1. Introduction: Was Einstein an Epistemological “Opportunist”?
• 2. Theoretical Holism: The Nature and Role of Conventions in Science
• 3. Simplicity and Theory Choice
• 4. Univocalness in the Theoretical Representation of Nature
• 5. Realism and Separability
• 6. The Principle Theories—Constructive Theories Distinction
• 7. Conclusion: Albert Einstein: Philosopher-Physicist
• Bibliography
  • Primary Literature: Einstein’s Work
  • Secondary Literature
• Academic Tools
• Other Internet Resources
• Related Entries

1. Introduction: Was Einstein an Epistemological “Opportunist”?

Late in 1944, Albert Einstein received a letter from Robert Thornton,a young African-American philosopher of science who had just finishedhis Ph.D. under Herbert Feigl at Minnesota and was beginning a new jobteaching physics at the University of Puerto Rico, Mayaguez. He hadwritten to solicit from Einstein a few supportive words on behalf ofhis efforts to introduce “as much of the philosophy of scienceas possible” into the modern physics course that he was to teachthe following spring (Thornton to Einstein, 28 November 1944, EA61–573). Here is what Einstein offered in reply:

I fully agree with you about the significance and educational value ofmethodology as well as history and philosophy of science. So manypeople today—and even professional scientists—seem to melike somebody who has seen thousands of trees but has never seen aforest. A knowledge of the historic and philosophical background givesthat kind of independence from prejudices of his generation from whichmost scientists are suffering. This independence created byphilosophical insight is—in my opinion—the mark ofdistinction between a mere artisan or specialist and a real seekerafter truth. (Einstein to Thornton, 7 December 1944, EA 61–574)

That Einstein meant what he said about the relevance of philosophy tophysics is evidenced by the fact that he had been saying more or lessthe same thing for decades. Thus, in a 1916 memorial note for ErnstMach, a physicist and philosopher to whom Einstein owed a special debt(see Norton 2010), he wrote:

How does it happen that a properly endowed natural scientist comes toconcern himself with epistemology? Is there no more valuable work inhis specialty? I hear many of my colleagues saying, and I sense itfrom many more, that they feel this way. I cannot share thissentiment. When I think about the ablest students whom I haveencountered in my teaching, that is, those who distinguish themselvesby their independence of judgment and not merely theirquick-wittedness, I can affirm that they had a vigorous interest inepistemology. They happily began discussions about the goals andmethods of science, and they showed unequivocally, through theirtenacity in defending their views, that the subject seemed importantto them. Indeed, one should not be surprised at this. (Einstein 1916,101)

How, exactly, does the philosophical habit of mind provide thephysicist with such “independence of judgment”? Einsteingoes on to explain:

Concepts that have proven useful in ordering things easily achievesuch an authority over us that we forget their earthly origins andaccept them as unalterable givens. Thus they come to be stamped as“necessities of thought,” “a priori givens,”etc. The path of scientific advance is often made impassable for along time through such errors. For that reason, it is by no means anidle game if we become practiced in analyzing the long commonplaceconcepts and exhibiting those circumstances upon which theirjustification and usefulness depend, how they have grown up,individually, out of the givens of experience. By this means, theirall-too-great authority will be broken. They will be removed if theycannot be properly legitimated, corrected if their correlation withgiven things be far too superfluous, replaced by others if a newsystem can be established that we prefer for whatever reason.(Einstein 1916, 102)

One is not surprised at Einstein’s then citing Mach’scritical analysis of the Newtonian conception of absolute space as aparadigm of what Mach, himself, termed the“historical-critical” method of philosophical analysis(Einstein 1916, 101, citing Ch. 2, §§ 6–7 ofMach’sMechanik, most likely the third edition, Mach1897).

The place of philosophy in physics was a theme to which Einsteinreturned time and again, it being clearly an issue of deep importanceto him. Sometimes he adopts a modest pose, as in this oft-quotedremark from his 1933 Spencer Lecture:

If you wish to learn from the theoretical physicist anything about themethods which he uses, I would give you the following piece of advice:Don’t listen to his words, examine his achievements. For to thediscoverer in that field, the constructions of his imagination appearso necessary and so natural that he is apt to treat them not as thecreations of his thoughts but as given realities. (Einstein 1933,5–6)

More typical, however, is the confident pose he struck three yearslater in “Physics and Reality”:

It has often been said, and certainly not without justification, thatthe man of science is a poor philosopher. Why then should it not bethe right thing for the physicist to let the philosopher do thephilosophizing? Such might indeed be the right thing at a time whenthe physicist believes he has at his disposal a rigid system offundamental concepts and fundamental laws which are so wellestablished that waves of doubt can not reach them; but it can not beright at a time when the very foundations of physics itself havebecome problematic as they are now. At a time like the present, whenexperience forces us to seek a newer and more solid foundation, thephysicist cannot simply surrender to the philosopher the criticalcontemplation of the theoretical foundations; for, he himself knowsbest, and feels more surely where the shoe pinches. In looking for anew foundation, he must try to make clear in his own mind just how farthe concepts which he uses are justified, and are necessities.(Einstein 1936, 349)

What kind of philosophy might we expect from thephilosopher-physicist? One thing that we should not expect from aphysicist who takes the philosophical turn in order to help solvefundamental physical problems is a systematic philosophy:

The reciprocal relationship of epistemology and science is ofnoteworthy kind. They are dependent upon each other. Epistemologywithout contact with science becomes an empty scheme. Science withoutepistemology is—insofar as it is thinkable atall—primitive and muddled. However, no sooner has theepistemologist, who is seeking a clear system, fought his way throughto such a system, than he is inclined to interpret the thought-contentof science in the sense of his system and to reject whatever does notfit into his system. The scientist, however, cannot afford to carryhis striving for epistemological systematic that far. He acceptsgratefully the epistemological conceptual analysis; but the externalconditions, which are set for him by the facts of experience, do notpermit him to let himself be too much restricted in the constructionof his conceptual world by the adherence to an epistemological system.He therefore must appear to the systematic epistemologist as a type ofunscrupulous opportunist: he appears asrealist insofar as heseeks to describe a world independent of the acts of perception; asidealist insofar as he looks upon the concepts and theoriesas free inventions of the human spirit (not logically derivable fromwhat is empirically given); aspositivist insofar as heconsiders his concepts and theories justifiedonly to theextent to which they furnish a logical representation of relationsamong sensory experiences. He may even appear asPlatonist orPythagorean insofar as he considers the viewpoint of logicalsimplicity as an indispensable and effective tool of his research.(Einstein 1949, 683–684)

But what strikes the “systematic epistemologist” as mereopportunism might appear otherwise when viewed from the perspective ofa physicist engaged, as Einstein himself put it, in “thecritical contemplation of the theoretical foundations.” Theoverarching goal of that critical contemplation was, for Einstein, thecreation of a unified foundation for physics after the model of afield theory like general relativity (see Sauer 2014 for non-technicaloverview on Einstein’s approach to the unified field theoryprogram). Einstein failed in his quest, but there was a consistencyand constancy in the striving that informed as well the philosophy ofscience developing hand in hand with the scientific project.

Indeed, from early to late a few key ideas played the central, leadingrole in Einstein’s philosophy of science, ideas about whichEinstein evinced surprisingly little doubt even while achieving anever deeper understanding of their implications. For the purposes ofthe following comparatively brief overview, we can confine ourattention to just five topics:

• Theoretical holism.
• Simplicity and theory choice.
• Univocalness in the theoretical representation of nature.
• Realism and separability.
• The principle theories-constructive theories distinction.

The emphasis on the continuity and coherence in the development ofEinstein’s philosophy of science contrasts with an account suchas Gerald Holton’s (1968), which claims to find a majorphilosophical break in the mid-1910s, in the form of a turn away froma sympathy for an anti-metaphysical positivism and toward a robustscientific realism. Holton sees this turn being driven byEinstein’s alleged realization that general relativity, bycontrast with special relativity, requires a realistic ontology.However, Einstein was probably never an ardent “Machian” positivist,^[1] and he was never a scientific realist, at least not in the senseacquired by the term “scientific realist” in latertwentieth century philosophical discourse (see Howard 1993). Einsteinexpected scientific theories to have the proper empirical credentials,but he was no positivist; and he expected scientific theories to givean account of physical reality, but he was no scientific realist.Moreover, in both respects his views remained more or less the samefrom the beginning to the end of his career.

Why Einstein did not think himself a realist (he said so explicitly)is discussed below. Why he is not to be understood as a positivistdeserves a word or two of further discussion here, if only because thebelief that he was sympathetic to positivism, at least early in hislife, is so widespread (for a fuller discussion, see Howard 1993).

That Einstein later repudiated positivism is beyond doubt. Manyremarks from at least the early 1920s through the end of his life makethis clear. In 1946 he explained what he took to be Mach’s basicerror:

He did not place in the correct light the essentially constructive andspeculative nature of all thinking and more especially of scientificthinking; in consequence, he condemned theory precisely at thosepoints where its constructive-speculative character comes to lightunmistakably, such as in the kinetic theory of atoms. (Einstein 1946,21)

Is Einstein here also criticizing his own youthful philosophicalindiscretions? The very example that Einstein gives here makes anysuch interpretation highly implausible, because one ofEinstein’s main goals in his early work on Brownian motion(Einstein 1905b) was precisely to prove the reality of atoms, this inthe face of the then famous skepticism of thinkers like Mach andWilhelm Ostwald:

My principal aim in this was to find facts that would guarantee asmuch as possible the existence of atoms of definite size.… Theagreement of these considerations with experience together withPlanck’s determination of the true molecular size from the lawof radiation (for high temperatures) convinced the skeptics, who werequite numerous at that time (Ostwald, Mach), of the reality of atoms.(Einstein 1946, 45, 47)

Why, then, is the belief in Einstein’s early sympathy forpositivism so well entrenched?

The one piece of evidence standardly cited for a youthful flirtationwith positivism is Einstein’s critique of the notion of absolutedistant simultaneity in his 1905 paper on special relativity (Einstein1905c). Einstein speaks there of “observers,” but in anepistemologically neutral way that can be replaced by talk of aninertial frame of reference. What really bothers Einstein aboutdistant simultaneity is not that it is observationally inaccessiblebut that it involves a two-fold arbitrariness, one in the choice of aninertial frame of reference and one in the stipulation within a givenframe of a convention regarding the ratio of the times required for alight signal to go from one stationary observer to another and backagain. Likewise, Einstein faults classical Maxwellian electrodynamicsfor an asymmetry in the way it explains electromagnetic inductiondepending on whether it is the coil or the magnet that is assumed tobe at rest. If the effect is the same—a current in thecoil—why, asks Einstein, should there be two differentexplanations: an electrical field created in the vicinity of a movingmagnet or an electromotive force induced in a conductor moving througha stationary magnetic field? To be sure, whether it is the coil or themagnet that is taken to be at rest makes no observable difference, butthe problem, from Einstein’s point of view, is the asymmetry inthe two explanations. Even the young Einstein was no positivist.

First generation logical empiricists sought to legitimate theirmovement in part by claiming Einstein as a friend. They may beforgiven their putting a forced interpretation on arguments taken outof context. We can do better.

Einstein’s philosophy of science is an original synthesisdrawing upon many philosophical resources, from neo-Kantianism toMachian empiricism and Duhemian conventionalism. Other thinkers andmovements, most notably the logical empiricists, drew upon the sameresources. But Einstein put the pieces together in a mannerimportantly different from Moritz Schlick, Hans Reichenbach, andRudolf Carnap, and he argued with them for decades about who was right(however much they obscured these differences in representing Einsteinpublicly as a friend of logical empiricism and scientific philosophy).Starting from the mid-1920s till the end of the decade Einstein showsome interest in the rationalistic realism of Émile Meyerson(Einstein, 1928; cf. Giovanelli 2018; on the contemporary debatebetween Einstein and Bergson, see Canales 2015). Understanding howEinstein puts those pieces together therefore sheds light not only onthe philosophical aspect of his own achievements in physics but alsoupon the larger history of the development of the philosophy ofscience in the twentieth century.

2. Theoretical Holism: The Nature and Role of Conventions in Science

Any philosophy of science must include an account of the relationbetween theory and evidence. Einstein learned about the historicity ofscientific concepts from Mach. But his preferred way of modeling thelogical relationship between theory and evidence was inspired mainlyby his reading of Pierre Duhem’sLa Théorie physique:son objet et sa structure (Duhem 1906). Einstein probably firstread Duhem, or at least learned the essentials of Duhem’sphilosophy of science around the fall of 1909, when, upon returning toZurich from the patent office in Bern to take up his first academicappointment at the University of Zurich, he became the upstairsneighbor of his old friend and fellow Zurich physics student,Friedrich Adler. Just a few months before, Adler had published theGerman translation ofLa Théorie physique (Duhem1908), and the philosophy of science became a frequent topic ofconversation between the new neighbors, Adler and Einstein (see Howard1990a).

Theoretical holism and the underdetermination of theory choice byempirical evidence are the central theses in Duhem’s philosophyof science (on Einstein’s and theory choice, see Oberheim 2016).His argument, in brief, is that at least in sciences like physics,where experiment is dense with sophisticated instrumentation whoseemployment itself requires theoretical interpretation, hypotheses arenot tested in isolation but only as part of whole bodies of theory. Itfollows that when there is a conflict between theory and evidence, thefit can be restored in a multiplicity of different ways. No statementis immune to revision because of a presumed status as a definition orthanks to some other a priori warrant, and most any statement can beretained on pain of suitable adjustments elsewhere in the total bodyof theory. Hence, theory choice is underdetermined by evidence.

That Einstein’s exposure to Duhem’s philosophy of sciencesoon left its mark is evident from lecture notes that Einsteinprepared for a course on electricity and magnetism at the Universityof Zurich in the winter semester of 1910/11. Einstein asks how one canassign a definite electrical charge everywhere within a material body,if the interior of the body is not accessible to test particles. A“Machian” positivist would deem such direct empiricalaccess necessary for meaningful talk of a charge distribution in theinterior of a sold. Einstein argues otherwise:

We have seen how experience led to the introd. of the concept of thequantity of electricity. it was defined by means of the forces thatsmall electrified bodies exert on each other. But now we extend theapplication of the concept to cases in which this definition cannot beapplied directly as soon as we conceive the el. forces as forcesexertedon electricity rather than on material particles. Weset up a conceptual system the individual parts of which do notcorrespond directly to empirical facts. Only a certain totality oftheoretical material corresponds again to a certain totality ofexperimental facts.

We find that such an el. continuum is always applicable only for therepresentation of el. states of affairs in the interior of ponderablebodies. Here too we define the vector of el. field strength as thevector of the mech. force exerted on the unit of pos. electr. quantityinside a body. But the force so defined is no longer directlyaccessible to exp. It is one part of a theoretical construction thatcan be correct or false, i.e., consistent or not consistent withexperience, onlyas a whole. (Collected Papers of AlbertEinstein, hereafter CPAE, Vol. 3, Doc. 11 [pp. 12–13])

One can hardly ask for a better summary of Duhem’s point of viewin application to a specific physical theory. Explicit citations ofDuhem by Einstein are rare (for details, see Howard 1990a). Butexplicit invocations of a holist picture of the structure andempirical interpretation of theories started to prevail at the turn ofthe 1920s.

During the decade 1905–1915, Einstein had more or lessexplicitly assumed that in a good theory there are certainindividual parts that can be directly coordinated with thebehavior of physically-existent objects used as probes. A theory canbe said to be ‘true or false’ if such objects respectivelybehave or do not behave as predicted. In special relativity, as inclassical mechanics, the fundamental geometrical/kinematicalvariables, the space and time coordinates, are measured with rods andclocks separately from the other non-geometrical variables, say,charge electric field strengths, which were supposed to be defined bymeasuring the force on a charge test particle. In general relativity,coordinates are no longer directly measurable independently from thegravitational field. Still, the line element \(ds\) (distance betweennearby spacetime points) was supposed to have a ‘natural’distance that can be measured with rods and clocks. In the late 1910s,pressed by the epistemological objections raised by differentinterlocutors—in particular Hermann Weyl (Ryckman 2005) and theyoung Wolfgang Pauli (Stachel, 2005)—Einstein was forced torecognize that this epistemological model was at most a provisionalcompromise. In principle rod- and clock-like structures should emergeas solutions of a future relativistic theory of matter, possibly afield theory encompassing gravitation and electromagnetism. In thiscontext, the sharp distinction between rods and clocks that serve todefine the geometrical/kinematical structure of the theory and othermaterial systems would become questionable. Einstein regarded suchdistinction as provisionally necessary, give the current state ofphysics. However, he recognized that in principle a physical theoryshould construct rods and clocks as solutions to its equations (seeRyckman 2017, ch. VII for an overview on Einstein view on the relationbetween geometry and experience).

Einstein addressed this issue in several popular writings during the1920s, in particular, the famous lectureGeometrie undErfahrung (Einstein 1921, see also Einstein, 1923, Einstein,1924, Einstein 1926; Einstein 1926; see Giovanelli 2014 for anoverview).Sub specie temporis, he argued, it was useful tocompare the geometrical/kinematical structures of the theory withexperience separately from the rest of physics.Sub specieaeterni, however, only geometry and physics taken together can besaid to be ‘true or false.’ This epistemological modelbecame more appropriate, while Einstein was moving beyond generalrelativity in the direction of theory unifying the gravitational andthe electromagnetic field. Einstein had to rely on progressively moreabstract geometrical structures which could not be defined in terms ofthe behavior of some physical probes. Thus, the use of such structureswas justified because of their role in the theory as a whole. In thesecond half of the 1920s, in correspondence with Reichenbach(Giovanelli 2017) and Meyerson (Giovanelli 2018), Einstein even deniedthat the very distinction between geometrical and non-geometrical ismeaningful (Lehmkuhl 2014, Giovanelli 2016, 2022).

A different, but especially interesting example of Einstein’sreliance on a form of theoretical holism is found in a review thatEinstein wrote in 1924 of Alfred Elsbach’sKant undEinstein (1924), one of the flood of books and articles thentrying to reconcile the Kant’s philosophy. Having asserted thatrelativity theory is incompatible with Kant’s doctrine of the apriori, Einstein explains why, more generally, he is not sympatheticwith Kant:

This does not, at first, preclude one’s holding at least to theKantianproblematic, as, e.g., Cassirer has done. I am evenof the opinion that this standpoint can be rigorously refuted by nodevelopment of natural science. For one will always be able to saythat critical philosophers have until now erred in the establishmentof the a priori elements, and one will always be able to establish asystem of a priori elements that does not contradict a given physicalsystem. Let me briefly indicate why I do not find this standpointnatural. A physical theory consists of the parts (elements) A, B, C,D, that together constitute a logical whole which correctly connectsthe pertinent experiments (sense experiences). Then it tends to be thecase that the aggregate of fewer than all four elements, e.g., A, B,D,without C, no longer says anything about theseexperiences, and just as well A, B, C without D. One is then free toregard the aggregate of three of these elements, e.g., A, B, C as apriori, and only D as empirically conditioned. But what remainsunsatisfactory in this is always thearbitrariness in thechoice of those elements that one designates as a priori,entirely apart from the fact that the theory could one day be replacedby another that replaces certain of these elements (or all four) byothers. (Einstein 1924, 1688–1689)

Einstein’s point seems to be that while one can always choose todesignate selected elements as a priori and, hence, non-empirical, noprinciple determines which elements can be so designated, and ourability thus to designate them derives from the fact that it is onlythe totality of the elements that possesses empirical content.

Much the same point could be made, and was made by Duhem himself (seeDuhem 1906, part 2, ch. 6, sects. 8 and 9), against those who wouldinsulate certain statements against empirical refutation by claimingfor them the status of conventional definitions. Edouard Le Roy (1901)had argued thus about the law of free fall. It could not be refuted byexperiment because it functioned as a definition of “freefall.” And Henri Poincaré (1901) said much the same aboutthe principles of mechanics more generally. As Einstein answered theneo-Kantians, so Duhem answered this species of conventionalist: Yes,experiment cannot refute, say, the law of free fall by itself, butonly because it is part of a larger theoretical whole that hasempirical content only as a whole, and various other elements of thatwhole could as well be said to be, alone, immune to refutation.

That Einstein should deploy against the neo-Kantians in the early1920s the argument that Duhem used against the conventionalism ofPoincaré and Le Roy is interesting from the point of view ofEinstein’s relationships with those who were leading thedevelopment of logical empiricism and scientific philosophy in the1920s, especially Schlick and Reichenbach. Einstein shared withSchlick and Reichenbach the goal of crafting a new form of empiricismthat would be adequate to the task of defending general relativityagainst neo-Kantian critiques (see Schlick 1917 and 1921, andReichenbach 1920, 1924, and 1928; for more detail, see Howard 1994a).But while they all agreed that what Kant regarded as the a priorielement in scientific cognition was better understood as aconventional moment in science, they were growing to disagreedramatically over the nature and place of conventions in science. Theclassic logical empiricist view that the moment of convention wasrestricted to conventional coordinating definitions that endowindividual primitive terms, worked well, but did not comport well withthe holism about theories

It was this argument over the nature and place of conventions inscience that underlies Einstein’s gradual philosophicalestrangement from Schlick and Reichenbach in the 1920s. Serious in itsown right, the argument over conventions was entangled with two otherissues as well, namely, realism and Einstein’s famous view oftheories as the “free creations of the human spirit” (see,for example, Einstein 1921). In both instances what troubled Einsteinwas that a verificationist semantics made the link between theory andexperience too strong, leaving too small a role for theory, itself,and the creative theorizing that produces it.

If theory choice is empirically determinate, especially if theoreticalconcepts are explicitly constructed from empirical primitives, as inCarnap’s program in theAufbau (Carnap 1928), then itis hard to see how theory gives us a story about anything other thanexperience. As noted, Einstein was not what we would today call ascientific realist, but he still believed that there was content intheory beyond mere empirical content (on the relations betweenEinstein’s realism and constructism see Ryckman 2017, ch. 8 and9). He believed that theoretical science gave us a window on natureitself, even if, in principle, there will be no one uniquely correctstory at the level of deep ontology (see below, section 5). And if theonly choice in theory choice is one among conventional coordinatingdefinitions, then that is no choice at all, a point stressed byReichenbach, especially, as an important positive implication of hisposition. Reichenbach argued that if empirical content is the onlycontent, then empirically equivalent theories have the same content,the difference resulting from their different choices of coordinatingdefinitions being like in kind to the difference between “esregnet” and “il pleut,” or the difference betweenexpressing the result of a measurement in English or metric units,just two different ways of saying the same thing. But then, Einsteinwould ask, where is there any role for the creative intelligence ofthe theoretical physicist if there is no room for genuine choice inscience, if experience somehow dictates theory construction?

The argument over the nature and role of conventions in sciencecontinued to the very end of Einstein’s life, reaching itshighest level of sophistication in the exchange between Reichenbachand Einstein the Library of Living Philosopher’s volume,Albert Einstein: Philosopher-Physicist (Schilpp 1949). Thequestion is, again, whether the choice of a geometry is empirical,conventional, or a priori. In his contribution, Reichenbach reassertedhis old view that once an appropriate coordinating definition isestablished, equating some “practically rigid rod” withthe geometer’s “rigid body,” then the geometry ofphysical space is wholly determined by empirical evidence:

The choice of a geometry is arbitrary only so long as no definition ofcongruence is specified. Once this definition is set up, it becomes anempirical questionwhich geometry holds for physicalspace.… The conventionalist overlooks the fact that only theincomplete statement of a geometry, in which a reference to thedefinition of congruence is omitted, is arbitrary. (Reichenbach 1949,297)

Einstein’s clever reply includes a dialogue between twocharacters, “Reichenbach” and“Poincaré,” in which “Reichenbach”concedes to “Poincaré” that there are no perfectlyrigid bodies in nature and that physics must be used to correct forsuch things as thermal deformations, from which it follows that whatwe actually test is geometry plus physics, not geometry alone. Here an“anonymous non-positivist” takes“Poincaré’s” place, out of respect, saysEinstein, “for Poincaré’s superiority as thinkerand author” (Einstein 1949, 677), but also, perhaps, because herealized that the point of view that follows was more Duhem thanPoincaré. The “non-positivist” then argues thatone’s granting that geometry and physics are tested togethercontravenes the positivist identification of meaning withverifiability:

Non-Positivist: If, under the stated circumstances, you holddistance to be a legitimate concept, how then is it with your basicprinciple (meaning = verifiability)? Must you not come to the pointwhere you deny the meaning of geometrical statements and concedemeaning only to the completely developed theory of relativity (whichstill does not exist at all as a finished product)? Must you not grantthat no “meaning” whatsoever, in your sense, belongs tothe individual concepts and statements of a physical theory, suchmeaning belonging instead to the whole system insofar as it makes“intelligible” what is given in experience? Why do theindividual concepts that occur in a theory require any separatejustification after all, if they are indispensable only within theframework of the logical structure of the theory, and if it is thetheory as a whole that stands the test? (Einstein 1949, 678).

Two years before the Quine’s publication of “Two Dogmas ofEmpiricism” (1951), Einstein here makes explicit the semanticimplications of a thoroughgoing holism.

If theory choice is empirically underdetermined, then an obviousquestion is why we are so little aware of the underdetermination inthe day-to-day conduct of science. In a 1918 address celebrating MaxPlanck’s sixtieth birthday, Einstein approached this questionvia a distinction between practice and principle:

The supreme task of the physicist is … the search for thosemost general, elementary laws from which the world picture is to beobtained through pure deduction. No logical path leads to theseelementary laws; it is instead just the intuition that rests on anempathic understanding of experience. In this state of methodologicaluncertainty one can think that arbitrarily many, in themselves equallyjustified systems of theoretical principles were possible; and thisopinion is,in principle, certainly correct. But thedevelopment of physics has shown that of all the conceivabletheoretical constructions a single one has, at any given time, proveditself unconditionally superior to all others. No one who has reallygone deeply into the subject will deny that, in practice, the world ofperceptions determines the theoretical system unambiguously, eventhough no logical path leads from the perceptions to the basicprinciples of the theory. (Einstein 1918, 31; Howard’stranslation)

But why is theory choice, in practice, seemingly empiricallydetermined? Einstein hinted at an answer the year before in a letterto Schlick, where he commended Schlick’s argument that the deepelements of a theoretical ontology have as much claim to the status ofthe real as do Mach’s elements of sensation (Schlick 1917), butsuggested that we are nonetheless speaking of two different kinds ofreality. How do they differ?

It appears to me that the word “real” is taken indifferent senses, according to whether impressions or events, that isto say, states of affairs in the physical sense, are spoken of.

If two different peoples pursue physics independently of one another,they will create systems that certainly agree as regards theimpressions (“elements” in Mach’s sense). The mentalconstructions that the two devise for connecting these“elements” can be vastly different. And the twoconstructions need not agree as regards the “events”; forthese surely belong to the conceptual constructions. Certainly on the“elements,” but not the “events,” are real inthe sense of being “given unavoidably in experience.”

But if we designate as “real” that which we arrange in thespace-time-schema, as you have done in the theory of knowledge, thenwithout doubt the “events,” above all, are real.…I would like to recommend a clean conceptual distinctionhere. (Einstein to Schlick, 21 May 1917, CPAE, Vol. 8, Doc.343)

Why, in practice, are physicists unaware of underdetermination? It isbecause ours is not the situation of “two different peoplespursu[ing] physics independently of one another.” ThoughEinstein does not say it explicitly, the implication seems to be thatapparent determination in theory choice is mainly a consequence of ourall being similarly socialized as we become members of a commonscientific community. Part of what it means to be a member of a such acommunity is that we have been taught to make our theoretical choicesin accord with criteria or values that we hold in common.

3. Simplicity and Theory Choice

For Einstein, as for many others, simplicity is the criterion thatmainly steers theory choice in domains where experiment andobservation no longer provide an unambiguous guide. This, too, is atheme sounded early and late in Einstein’s philosophicalreflections (for more detail, see Howard 1998, Norton 2000, van Dongen2002, 2010, 2017 Giovanelli 2018). For example, the just-quoted remarkfrom 1918 about the apparent determination of theory choice inpractice, contrasted with in-principle underdeterminationcontinues:

Furthermore this conceptual system that is univocally coordinated withthe world of experience is reducible to a few basic laws from whichthe whole system can be developed logically. With every new importantadvance the researcher here sees his expectations surpassed, in thatthose basic laws are more and more simplified under the press ofexperience. With astonishment he sees apparent chaos resolved into asublime order that is to be attributed not to the rule of theindividual mind, but to the constitution of the world of experience;this is what Leibniz so happily characterized as“pre-established harmony.” Physicists strenuously reproachmany epistemologists for their insufficient appreciation of thiscircumstance. Herein, it seems to me, lie the roots of the controversycarried on some years ago between Mach and Planck. (Einstein 1918, p.31)

There is more than a little autobiography here, for as Einsteinstressed repeatedly in later years, he understood the success of hisown quest for a general theory of relativity as a result of hisseeking the simplest set of field equations satisfying a given set ofconstraints.

Einstein’s celebration of simplicity as a guide to theory choicecomes clearly to the fore in the early 1930s, when he was immersed hisproject of a unified field theory (see, van Dongen 2010 for areconstruction of the philosophical underpinning of Einstein’ssearch of a unified field theory). Witness what he wrote in his 1933Herbert Spencer lecture:

If, then, it is true that the axiomatic foundation of theoreticalphysics cannot be extracted from experience but must be freelyinvented, may we ever hope to find the right way? Furthermore, doesthis right way exist anywhere other than in our illusions? May we hopeto be guided safely by experience at all, if there exist theories(such as classical mechanics) which to a large extent do justice toexperience, without comprehending the matter in a deep way?

To these questions, I answer with complete confidence, that, in myopinion, the right way exists, and that we are capable of finding it.Our experience hitherto justifies us in trusting that nature is therealization of the simplest that is mathematically conceivable. I amconvinced that purely mathematical construction enables us to findthose concepts and those lawlike connections between them that providethe key to the understanding of natural phenomena. Useful mathematicalconcepts may well be suggested by experience, but in no way can theybe derived from it. Experience naturally remains the sole criterion ofthe usefulness of a mathematical construction for physics. But theactual creative principle lies in mathematics. Thus, in a certainsense, I take it to be true that pure thought can grasp the real, asthe ancients had dreamed. (Einstein 1933, p. 183; Howard’stranslation)

Einstein’s conviction that the theoretical physicist must trustsimplicity is that his work was moving steadily into domains everfurther removed from direct contact with observation and experiment.Einstein started to routinely claim that this was the lesson he haddrawn from the way in which he had found general relativity (Norton2000). There are, however, good reasons to think that Einstein’sselective recollections (Jannsen and Renn 2007) were instrumental tohis defense of relying on a purely mathematical strategy in the searchfor a unified field theory (van Dongen 2010):

The theory of relativity is a beautiful example of the basic characterof the modern development of theory. That is to say, the hypothesesfrom which one starts become ever more abstract and more remote fromexperience. But in return one comes closer to the preeminent goal ofscience, that of encompassing a maximum of empirical contents throughlogical deduction with a minimum of hypotheses or axioms. Theintellectual path from the axioms to the empirical contents or to thetestable consequences becomes, thereby, ever longer and more subtle.The theoretician is forced, ever more, to allow himself to be directedby purely mathematical, formal points of view in the search fortheories, because the physical experience of the experimenter is notcapable of leading us up to the regions of the highest abstraction.Tentative deduction takes the place of the predominantly inductivemethods appropriate to the youthful state of science. Such atheoretical structure must be quite thoroughly elaborated in order forit to lead to consequences that can be compared with experience. It iscertainly the case that here, as well, the empirical fact is theall-powerful judge. But its judgment can be handed down only on thebasis of great and difficult intellectual effort that first bridgesthe wide space between the axioms and the testable consequences. Thetheorist must accomplish this Herculean task with the clearunderstanding that this effort may only be destined to prepare the wayfor a death sentence for his theory. One should not reproach thetheorist who undertakes such a task by calling him a fantast; instead,one must allow him his fantasizing, since for him there is no otherway to his goal whatsoever. Indeed, it is no planless fantasizing, butrather a search for the logically simplest possibilities and theirconsequences. (Einstein 1954, 238–239; Howard’stranslation)

What warrant is there for thus trusting in simplicity? At best one cando a kind of meta-induction. That “the totality of all sensoryexperience can be ‘comprehended’ on the basis of aconceptual system built on premises of great simplicity” will bederided by skeptics as a “miracle creed,” but, Einsteinadds, “it is a miracle creed which has been borne out to anamazing extent by the development of science” (Einstein 1950, p.342). The success of previous physical theories justifies our trustingthat nature is the realization of the simplest that is mathematicallyconceivable

But for all that Einstein’s faith in simplicity was strong, hedespaired of giving a precise, formal characterization of how weassess the simplicity of a theory. In 1946 he wrote about theperspective of simplicity (here termed the “innerperfection” of a theory):

This point of view, whose exact formulation meets with greatdifficulties, has played an important role in the selection andevaluation of theories from time immemorial. The problem here is notsimply one of a kind of enumeration of the logically independentpremises (if anything like this were at all possible withoutambiguity), but one of a kind of reciprocal weighing ofincommensurable qualities.… I shall not attempt to excuse thelack of precision of [these] assertions … on the grounds ofinsufficient space at my disposal; I must confess herewith that Icannot at this point, and perhaps not at all, replace these hints bymore precise definitions. I believe, however, that a sharperformulation would be possible. In any case it turns out that among the“oracles” there usually is agreement in judging the“inner perfection” of the theories and even more soconcerning the degree of “external confirmation.”(Einstein 1946, pp. 21, 23).

As in 1918, so in 1946 and beyond, Einstein continues to be impressedthat the “oracles,” presumably the leaders of the relevantscientific community, tend to agree in their judgments of simplicity.That is why, in practice, simplicity seems to determine theory choiceunivocally.

4. Univocalness in the Theoretical Representation of Nature

In the physics and philosophy of science literature of the latenineteenth and early twentieth centuries, the principle according towhich scientific theorizing should strive for a univocalrepresentation of nature was widely and well known under the name thatit was given in the title of a widely-cited essay by Joseph Petzoldt,“The Law of Univocalness” [“Das Gesetz derEindeutigkeit”] (Petzoldt 1895). An indication that the map ofphilosophical positions was drawn then in a manner very different fromtoday is to found in the fact that this principle found favor amongboth anti-metaphysical logical empiricists, such as Carnap, andneo-Kantians, such as Cassirer. It played a major role in debates overthe ontology of general relativity and was an important part of thebackground to the development of the modern concept of categoricity informal semantics (for more on the history, influence, and demise ofthe principle of univocalness, see Howard 1992 and 1996). One can findno more ardent and consistent champion of the principle thanEinstein.

The principle of univocalness should not be mistaken for a denial ofthe underdetermination thesis. The latter asserts that a multiplicityof theories can equally well account for a given body of empiricalevidence, perhaps even the infinity of all possible evidence in theextreme, Quinean version of the thesis. The principle of univocalnessasserts (in a somewhat anachronistic formulation) that any one theory,even any one among a set of empirically equivalent theories, shouldprovide a univocal representation of nature by determining for itselfan isomorphic set of models. The unambiguous determination of theorychoice by evidence is not the same thing as the univocal determinationof a class of models by a theory.

The principle of univocalness played a central role inEinstein’s struggles to formulate the general theory ofrelativity. When, in 1913, Einstein wrongly rejected a fully generallycovariant theory of gravitation, he did so in part because he thought,wrongly, that generally covariant field equations failed the test ofunivocalness. More specifically, he reasoned wrongly that for a regionof spacetime devoid of matter and energy—a“hole”—generally covariant field equations permitthe construction of two different solutions, different in the sensethat, in general, for spacetime points inside the hole, they assigndifferent values of the metric tensor to one and the same point (formore on the history of this episode, see Stachel 1980 and Norton1984). But Einstein’s “hole argument” is wrong, andhis own diagnosis of the error in 1915 rests again, ironically, on adeployment of the principle of univocalness. What Einstein realized in1915 was that, in 1913, he was wrongly assuming that a coordinatechart sufficed to fix the identity of spacetime manifold points. Theapplication of a coordinate chart cannot suffice to individuatemanifold points precisely because a coordinate chart is not aninvariant labeling scheme, whereas univocalness in the representationof nature requires such invariance (see Howard and Norton 1993 andHoward 1999 for further discussion).

Here is how Einstein explained his change of perspective in a letterto Paul Ehrenfest of 26 December 1915, just a few weeks after thepublication of the final, generally covariant formulation of thegeneral theory of relativity:

In §12 of my work of last year, everything is correct (in thefirst three paragraphs) up to that which is printed with emphasis atthe end of the third paragraph. From the fact that the two systems\(G(x)\) and \(G'(x)\), referred to the same reference system, satisfythe conditions of the grav. field, no contradiction follows with theunivocalness of events. That which was apparently compelling in thesereflections founders immediately, if one considers that

1. the reference system signifies nothing real
2. that the (simultaneous) realization of two different \(g\)-systems(or better, two different grav. fields) in the same region of thecontinuum is impossible according to the nature of the theory.

In place of §12, the following reflections must appear. Thephysically real in the universe of events (in contrast to that whichis dependent upon the choice of a reference system) consists inspatiotemporal coincidences.* [Footnote *: and in nothingelse!] Real are, e.g., the intersections of two different world lines,or the statement that theydo not intersect. Those statementsthat refer to the physically real therefore do not founder on anyunivocal coordinate transformation. If two systems of the \(g_{\muv}\) (or in general the variables employed in the description of theworld) are so created that one can obtain the second from the firstthrough mere spacetime transformation, then they are completelyequivalent. For they have all spatiotemporal point coincidences incommon, i.e., everything that is observable.

These reflections show at the same time how natural the demand forgeneral covariance is. (CPAE, Vol. 8, Doc. 173)

Einstein’s new point of view, according to which the physicallyreal consists exclusively in that which can be constructed on thebasis of spacetime coincidences, spacetime points, for example, beingregarded as intersections of world lines, is now known as the“point-coincidence argument.” (Giovanelli 2021). Einsteinmight have been inspired by a paper by the young mathematician ErichKretschmann (Howard and Norton 1993; cf. Giovanelli 2013) or possiblyby a conversation with Schlick (Engler and Renn, 2017). Spacetimecoincidences play this privileged ontic role because they areinvariant and, thus, univocally determined. Spacetimecoordinates lack such invariance, a circumstance thatEinstein thereafter repeatedly formulated as the claim that space andtime “thereby lose the last vestige of physical reality”(see, for example, Einstein to Ehrenfest, 5 January 1916, CPAE, Vol.8, Doc. 180).

One telling measure of the philosophical importance ofEinstein’s new perspective on the ontology of spacetime is thefact that Schlick devoted his first book,Raum und Zeit in dengegenwärtigen Physik (1917), a book for which Einstein hadhigh praise (see Howard 1984 and 1999). But what most interestedEinstein was Schlick’s discussion of the reality concept.Schlick argued that Mach was wrong to regard only the elements ofsensation as real. Spacetime events, individuated invariantly asspacetime coincidences, have as much or more right to be taken asreal, precisely because of the univocal manner of their determination.Einstein wholeheartedly agreed, though he ventured the above-quotedsuggestion that one should distinguish the two kinds ofreality—that of the elements and that of the spacetimeevents—on the ground that if “two different peoples”pursued physics independently of one another they were fated to agreeabout the elements but would almost surely produce differenttheoretical constructions at the level of the spacetime eventontology. Note, again, that underdetermination is not a failure ofunivocalness. Different though they will be, each people’stheoretical construction of an event ontology would be expected to beunivocal.

Schlick, of course, went on to become the founder of the ViennaCircle, a leading figure in the development of logical empiricism, achampion of verificationism. That being so, an important questionarises about Schlick’s interpretation of Einstein on theunivocal determination of spacetime events as spacetime coincidences.The question is this: Do such univocal coincidences play such aprivileged role because of their reality or because of theirobservability. Clearly the former—the reality of that which isunivocally determined—is important. But are univocal spacetimecoincidences real because, thanks to their invariance, they areobservable? Or is their observability consequent upon their invariantreality? Einstein, himself, repeatedly stressed the observablecharacter of spacetime coincidences, as in the 26 December 1915 letterto Ehrenfest quoted above (for additional references and a fullerdiscussion, see Howard 1999).^[2]

Schlick, still a self-described realist in 1917, was clear about therelationship between observability and reality. He distinguishedmacroscopic coincidences in the field of our sense experience, towhich he does accord a privileged and foundational epistemic status,from the microscopic point coincidences that define an ontology ofspacetime manifold points. Mapping the former onto the latter is, forSchlick, an important part of the business of confirmation, but thereality of the spacetime manifold points is in no way consequent upontheir observability. Indeed, how, strictly speaking, can one even talkof the observation ofinfinitesimal spacetime coincidences ofthe kind encountered in the intersection of two world lines? In fact,the order of implication goes the other way: Spacetime eventsindividuated as spacetime coincidences are real because they areinvariant, and such observability as they might possess is consequentupon their status as invariant bits of physical reality. For Einstein,and for Schlick in 1917, understanding the latter—physicalreality—is the goal of physical theory.

5. Realism and Separability

As we have seen, Schlick’sRaum und Zeit in dengegenwärtigen Physik promoted a realistic interpretation ofthe ontology of general relativity. After reading the manuscript earlyin 1917, Einstein wrote to Schlick on 21 May that “the lastsection ‘Relations to Philosophy’ seems to meexcellent” (CPAE, Vol. 8, Doc. 343), just the sort of praise onewould expect from a fellow realist. Three years earlier, the Bonnmathematician, Eduard Study, had written another well-known, indeedvery well-known defense of realism,Die realistische Weltansichtund die Lehre vom Raume (1914). Einstein read it in September of1918. Much of it he liked, especially the droll style, as he said toStudy in a letter of 17 September (CPAE, Vol. 8, Doc. 618). Pressed byStudy to say more about the points where he disagreed, Einsteinreplied on 25 September in a rather surprising way:

I am supposed to explain to you my doubts? By laying stress on theseit will appear that I want to pick holes in you everywhere. But thingsare not so bad, because I do not feel comfortable and at home in anyof the “isms.” It always seems to me as though such an ismwere strong only so long as it nourishes itself on the weakness of itcounter-ism; but if the latter is struck dead, and it is alone on anopen field, then it also turns out to be unsteady on its feet.So,away we go!

“The physical world is real.” That is supposed to be thefundamental hypothesis. What does “hypothesis” mean here?For me, a hypothesis is a statement, whosetruth must beassumed for the moment,but whose meaning must be raised above allambiguity. The above statement appears to me, however, to be, initself, meaningless, as if one said: “The physical world iscock-a-doodle-doo.” It appears to me that the “real”is an intrinsically empty, meaningless category (pigeon hole), whosemonstrous importance lies only in the fact that I can do certainthings in it and not certain others. This division is, to be sure, notanarbitrary one, but instead ….

I concede that the natural sciences concern the “real,”but I am still not a realist. (CPAE, Vol. 8, Doc. 624)

Lest there be any doubt that Einstein has little sympathy for theother side, he adds:

The positivist or pragmatist is strong as long as he battles againstthe opinion that there [are] concepts that are anchored in the“A priori.” When, in his enthusiasm, [he] forgets that allknowledge consists [in] concepts and judgments, then that is aweakness that lies not in the nature of things but in his personaldisposition just as with the senseless battle against hypotheses, cf.the clear book by Duhem. In any case, the railing against atoms restsupon this weakness. Oh, how hard things are for man in this world; thepath to originality leads through unreason (in the sciences), throughugliness (in the arts)-at least the path that many find passable.(CPAE, Vol. 8, Doc. 624)

What could Einstein mean by saying that he concedes that the naturalsciences concern the “real,” but that he is “stillnot a realist” and that the “real” in the statement,“the physical world is real,” is an “intrinsicallyempty, meaningless category”?

The answer might be that realism, for Einstein, is not a philosophicaldoctrine about the interpretation of scientific theories or thesemantics of theoretical terms.^[3] For Einstein, realism is a physical postulate, one of a mostinteresting kind, as he explained on 18 March 1948 in a long note atthe end of the manuscript of Max Born’s Waynflete Lectures,Natural Philosophy of Cause and Chance (1949), which Born hadsent to Einstein for commentary:

I just want to explain what I mean when I say that we should try tohold on to physical reality. We are, to be sure, all of us aware ofthe situation regarding what will turn out to be the basicfoundational concepts in physics: the point-mass or the particle issurely not among them; the field, in the Faraday/Maxwell sense,might be, but not with certainty. But that which we conceive asexisting (’actual’) should somehow be localized in timeand space. That is, the real in one part of space, A, should (intheory) somehow ‘exist’ independently of that which isthought of as real in another part of space, B. If a physical systemstretches over the parts of space Aand B, then what ispresent in B should somehow have an existence independent of what ispresent in A. What is actually present in B should thus not dependupon the type of measurement carried out in the part of space, A; itshould also be independent of whether or not, after all, a measurementis made in A.

If one adheres to this program, then one can hardly view thequantum-theoretical description as acomplete representationof the physically real. If one attempts, nevertheless, so to view it,then one must assume that the physically real in B undergoes a suddenchange because of a measurement in A. My physical instincts bristle atthat suggestion.

However, if one renounces the assumption that what is present indifferent parts of space has an independent, real existence, then I donot at all see what physics is supposed to describe. For what isthought to by a ‘system’ is, after all, just conventional,and I do not see how one is supposed to divide up the worldobjectively so that one can make statements about the parts. (Born1969, 223–224; Howard’s translation)

Realism is thus the thesis of spatial separability, the claim thatspatial separation is a sufficient condition for the individuation ofphysical systems, and its assumption is here made into almost anecessary condition for the possibility of an intelligible science ofphysics.

The postulate of spatial separability as that which undergirds theontic independence and, hence, individual identities of the systemsthat physics describes was an important part of Einstein’sthinking about the foundations of physics since at least the time ofhis very first paper on the quantum hypothesis in 1905 (Einstein1905a; for more detail on the early history of this idea inEinstein’s thinking, see Howard 1990b and Bacciagaluppi andCrull 2024). But the true significance of the separability principleemerged most clearly in 1935, when (as hinted in the just-quotedremark) Einstein made it one of the central premises of his argumentfor the incompleteness of quantum mechanics (see Howard 1985 and1989). It is not so clearly deployed in the published version of theEinstein, Podolsky, Rosen paper (1935), but Einstein did not writethat paper and did not like the way the argument appeared there.Separability is, however, an explicit premise in all ofEinstein’s later presentations of the argument for theincompleteness of quantum mechanics, both in correspondence and inprint (see Howard 1985 for a detailed list of references).

In brief, the argument is this. Separability implies that spacelikeseparated systems have associated with them independent real states ofaffairs. A second postulate, locality, implies that the events in oneregion of spacetime cannot physically influence physical reality in aregion of spacetime separated from the first by a spacelike interval.Consider now an experiment in which two systems, A and B, interact andseparate, subsequent measurements on each corresponding to spacelikeseparated events. Separability implies that A and B have separate realphysical states, and locality implies that the measurement performedon A cannot influence B’s real physical state. But quantummechanics ascribes different theoretical states, different wavefunctions, to B depending upon that parameter that is measured on A.Therefore, quantum mechanics ascribes different theoretical states toB, when B possesses, in fact, one real physical state. Hence quantummechanics is incomplete.

One wants to ask many questions. First, what notion of completeness isbeing invoked here? It is not deductive completeness. It is closer inkind to what is termed “categoricity” in formal semantics,a categorical theory being one whose models are all isomorphic to oneanother. It is closer still to the principle discussed above—andcited as a precursor of the concept of categoricity—namely, theprinciple of univocalness, which we found doing such important work inEinstein’s quest for a general theory of relativity, where itwas the premise forcing the adoption of an invariant and thus univocalscheme for the individuation of spacetime manifold points.

The next question is why separability is viewed by Einstein asvirtually an a priori necessary condition for the possibility of ascience of physics. One reason is because a field theory like generalrelativity, which was Einstein’s model for a future unifiedfoundation for physics, is an extreme embodiment of the principle ofseparability: “Field theory has carried out this principle tothe extreme, in that it localizes within infinitely small(four-dimensional) space-elements the elementary things existingindependently of the one another that it takes as basic, as well asthe elementary laws it postulates for them” (Einstein 1948,321–322). And a field theory like general relativity can do thisbecause the infinitesimal metric interval—the careful way tothink about separation in general relativistic spacetime—isinvariant (hence univocally determined) under all continuouscoordinate transformations.

Another reason why Einstein would be inclined to view separability asan a priori necessity is that, in thus invoking separability to groundindividuation, Einstein places himself in a tradition of so viewingspatial separability with very strong Kantian roots (and, before Kant,Newtonian roots), a tradition in which spatial separability was knownby the name that Arthur Schopenhauer famously gave to it, theprincipium individuationis (for a fuller discussion of thishistorical context, see Howard 1997).

A final question one wants to ask is: “What does any of thishave to do with realism?” One might grant Einstein’s pointthat a real ontology requires a principle of individuation withoutagreeing that separability provides the only conceivable suchprinciple. Separability together with the invariance of theinfinitesimal metric interval implies that, in a general relativisticspacetime, there are joints everywhere, meaning that we can carve upthe universe in any way we choose and still have ontically independentparts. But quantum entanglement can be read as implying that thislibertarian scheme of individuation does not work. Can quantummechanics not be given a realistic interpretation? Many would say,“yes.” Einstein said, “no.”

6. The Principle Theories—Constructive Theories Distinction

There is much that is original in Einstein’s philosophy ofscience as described thus far. At the very least, he rearranged thebits and pieces of doctrine that he learned from others—Kant,Mach, Duhem, Poincaré, Schlick, and others—in astrikingly novel way. But Einstein’s most original contributionto twentieth-century philosophy of science lies elsewhere, in hisdistinction between what he termed “principle theories”and “constructive theories.” (Giovanelli 2020, 2023)

This idea first found its way into print in a brief 1919 article intheTimes of London (Einstein 1919). A constructive theory,as the name implies, provides a constructive model for the phenomenaof interest. An example would be kinetic theory. A principle theoryconsists of a set of individually well-confirmed, high-level empiricalgeneralizations, “which permit of precise formulation”(Einstein 1914, 749). Examples include the first and second laws ofthermodynamics. Ultimate understanding requires a constructive theory,but often, says Einstein, progress in theory is impeded by prematureattempts at developing constructive theories in the absence ofsufficient constraints by means of which to narrow the range ofpossible constructive theories. It is the function of principletheories to provide such constraint, and progress is often bestachieved by focusing first on the establishment of such principles.According to Einstein, that is how he achieved his breakthrough withthe theory of relativity, which, he says, is a principle theory, itstwo principles being the relativity principle and the lightprinciple.

While the principle theories-constructive theories distinction firstmade its way into print in 1919, there is considerable evidence thatit played an explicit role in Einstein’s thinking much earlier(Einstein 1907, Einstein to Sommerfeld 14 January 1908, CPAE, vol. 5,Doc. 73, Einstein 1914). Nor was it only the relativity and lightprinciples that served Einstein as constraints in his theorizing.Thus, he explicitly mentions also the Boltzmann principle, \(S = k\log W\), as another such:

This equation connects thermodynamics with the molecular theory. Ityields, as well, the statistical probabilities of the states ofsystems for which we are not in a position to construct amolecular-theoretical model. To that extent, Boltzmann’smagnificent idea is of significance for theoretical physics …because it provides a heuristic principle whose range extends beyondthe domain of validity of molecular mechanics. (Einstein 1915, p.262).

Einstein is here alluding the famous entropic analogy whereby, in his1905 photon hypothesis paper, he reasoned from the fact that blackbody radiation in the Wien regime satisfied the Boltzmann principle tothe conclusion that, in that regime, radiation behaved as if itconsisted of mutually independent, corpuscle-like quanta ofelectromagnetic energy. The quantum hypothesis is a constructive modelof radiation; the Boltzmann principle is the constraint that firstsuggested that model.

There are anticipations of the principle theories-constructivetheories distinction in the nineteenth-century electrodynamicsliterature, James Clerk Maxwell, in particular, being a source fromwhich Einstein might well have drawn (see Harman 1998). At the turn ofthe century, the “physics of principles” was a subjectunder wide discussion. At the turn of 1900, Hendrik A. Lorentz(Lorentz 1900, 1905; see Frisch 2005) and Henri Poincaré (forexample, Poincaré 1904; see, Giedymin 1982, Darrigol 1995)presented the opposition between the “physics ofprinciples” and the “physics of models” ascommonplace. In a similar vein, Arnold Sommerfeld opposed a“physics of problems”, a style of doing physics based onconcrete puzzle solving, to the “practice of principles”defended by Max Planck (Seth 2010). Philipp Frank (1908, relying onRey 1909) defined relativity theory as a “ conceptualtheory” based on abstract, but empirically well confirmedprinciples rather than on intuitive models. Probably many otherexamples could be find. . But however extensive his borrowings (noexplicit debt was ever acknowledged), in Einstein’s hands thedistinction becomes a methodological tool of impressive scope andfertility. What is puzzling, and even a bit sad, is that this mostoriginal methodological insight of Einstein’s had comparativelylittle impact on later philosophy of science or practice in physics.Only in recent decades, Einstein constructive-principle distinctionhas attracted interest in the philosophical literature, originating astill living philosophical debate on the foundation of spacetimetheories (Brown 2005, Janssen 2009, Lange 2014).^[4]

7. Conclusion: Albert Einstein: Philosopher-Physicist

Einstein’s influence on twentieth-century philosophy of scienceis comparable to his influence on twentieth-century physics (Howard2014). What made that possible? One explanation looks to theinstitutional and disciplinary history of theoretical physics and thephilosophy of science. Each was, in its own domain, a new mode ofthought in the latter nineteenth century, and each finally began tosecure for itself a solid institutional basis in the early twentiethcentury. In a curious way, the two movements helped one another.Philosophers of science helped to legitimate theoretical physics bylocating the significant cognitive content of science in its theories.Theoretical physicists helped to legitimate the philosophy of scienceby providing for analysis a subject matter that was radicallyreshaping our understanding of nature and the place of humankindwithin it. In some cases the help was even more direct, as with thework of Einstein and Max Planck in the mid-1920s to create in thephysics department at the University of Berlin a chair in thephilosophy of science for Reichenbach (see Hecht and Hartmann 1982).And we should remember the example of the physicists Mach and LudwigBoltzmann who were the first two occupants of the new chair for thephilosophy of science at the University of Vienna at the turn of thecentury.

Another explanation looks to the education of young physicists inEinstein’s day. Not only was Einstein’s own youthfulreading heavily focused on philosophy, more generally, and thephilosophy of science, in particular (for an overview, see Einstein1989, xxiv–xxv; see also Howard 1994b), in which respect he wasnot unlike other physicists of his generation, but also his universityphysics curriculum included a required course on “The Theory ofScientific Thought” (see Einstein 1987, Doc. 28). An obviousquestion is whether or not the early cultivation of a philosophicalhabit of mind made a difference in the way Einstein and hiscontemporaries approached physics. As indicated by his November 1944letter to Robert Thorton quoted at the beginning of this article,Einstein thought that it did.

Bibliography

Primary Literature: Einstein’s Work

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