The Large Scale Structure of Logical Empiricism:
Unity of Science and the Elimination of Metaphysics

Greg Frost-Arnold

Abstract:  Two central and well-known philosophical goals of the logical empiricists are the unification of science and the elimination of metaphysics.  I argue, via textual analysis, that these two apparently distinct planks of the logical empiricist party platform are actually intimately related.  From the 1920’s through 1950, one abiding criterion for judging whether an apparently declarative assertion or descriptive term is metaphysical is that that assertion or term cannot be incorporated into a language of unified science.  I explore various versions of this criterion throughout the works of Carnap and Neurath.

University of Pittsburgh
History and Philosophy of Science Department
ggfst@pitt.edu

1. Introduction
Two central planks of the logical empiricists’ party platform are the unity of science thesis and their rejection of metaphysics.  To most current minds, these two topics likely appear prima facie rather different.   However, the central contention in this paper is that these two ideas are intimately intertwined in the writings of many logical empiricists. Close attention to the writings of central logical empiricists on the unity of science and the elimination of metaphysics reveals that, metaphorically speaking, these two goals are two sides of the same coin.  More prosaically, in different logical empiricists, from the 1920’s through 1950, we find the following criterion (or an approximation thereof) at work for detecting metaphysics: an apparently meaningful, contentual utterance is metaphysical if and only if it cannot be incorporated into ‘unified science’ [Einheitswissenschaft].  In this essay, I will focus on Carnap and Neurath, for they wrote most extensively on both unity of science and the elimination of metaphysics, and their work is prominent among both their peers and modern scholars re-evaluating logical empiricism.

2. Unity of Science 
What did the logical empiricists mean by the phrase ‘unity of science’? The fundamental fact to appreciate is that the unity of which the logical empiricists speak is not unity of laws or theories, but rather unity of language.  This point has been increasingly stressed by scholars, e.g. (Creath 1996), so I will not attempt a complete substantiation of this claim.  Nonetheless, I devote this section to an abbreviated elaboration and defense of this exegetical contention.  First, to be explicit, Carnap, Neurath and others stress repeatedly that their thesis is not that the results of biology, psychology, sociology etc. can (or will) be ultimately derived from a single fundamental theory (presumably physics).  Rather, the logical empiricists’ aim is to construct a language that can simultaneously express biological, psychological, social, and physical claims.  Carnap emphasizes that the reduction of (e.g.) biological laws to chemical or physical laws is an open question: “there is at present no unity of laws… On the other hand, there is a unity of language in science, viz., a common reduction basis for the terms of all branches of science” (1938, 61).  Neurath’s views are similar.  He does not demand a unity of laws: “Having a Universal Jargon [his term for his language of unified science] in common does not imply that the same scientific ‘laws’ have to be valid in the various fields of scientific research” (1946, 81).  Neurath, the social scientist, stresses the autonomy of sociological laws: “Comprehensive sociological laws can be found without the need to be able… to build up these sociological laws from physical ones” (1983, 75).
Neurath is more antagonistic than Carnap to this unification of theories or laws.  Neurath claims this need to fit all knowledge into a single Procrustean bed constitutes a fundamental error of Cartesian and Leibnizean rationalism, and he stresses that the model for unified science is not a system, but an encyclopedia: the claims of an encyclopedia, unlike the claims of a system, are not all derivable from a few precise axioms.  For example, in the first article in the International Encyclopedia of Unified Science, Neurath (the Encyclopedia’s editor-in-chief) writes: “the great French Encyclopedia,” whose work the new Encyclopedia continues, “was not a ‘faute de mieux encyclopedia’ in place of a comprehensive system, but an alternative to systems” (1938, 7; cf. 2, 16, 20).  This rejection of the single axiomatized system of knowledge in favor of a loosely connected encyclopedia is a leitmotif running throughout Neurath’s corpus.
What the logical empiricists’ unified science requires is not a unity of laws, but something weaker: unity of language.  We saw Carnap explicitly state this in his quote immediately above.  For Neurath as well, the crucial kind of unity is a unity of language: “We can use the everyday language which we use when we talk about cows and calves throughout our empiricist discussions.  This was for me the main element of ‘unity’” (1983, 233).  Philipp Frank provides perhaps the simplest formulation of the unity of science thesis: “there is one and the same language in all fields” of science (1947, 165).  In Logical Syntax, Carnap offers the following more precise characterization of the thesis: every sub-language of science can be translated without loss into one language (1934/1937, 320).  
The next question to ask is: which language or languages fit this bill?  Many logical empiricists agree that the physicalist language is one such.  In Logical Syntax, Carnap states that the thesis of physicalism is precisely that the physicalist language can successfully perform this function of an overarching language for all of science (1934/1937, 320).   Carnap defends this thesis most strongly in (1932/1934).  Neurath provides a detailed description of the physicalist language, which he also calls ‘universal jargon.’  It is not restricted to the vocabulary of physics.  Neurath describes his Universal Jargon as “an everyday language that avoids certain phrases and is enriched by certain other phrases” (1983, 208); it ‘avoids’ metaphysical terms, and ‘is enriched’ by technical terms (1983, 91-92).  But the fact that the physicalist language can serve as the language for unified science does not imply that no other languages could. 
Finally, the logical empiricist unity of science thesis is not refuted by Suppes’ observation (1978, 5) that the actual terminology used in various sub-disciplines of the sciences is increasingly divergent, with each subfield developing its own jargon.  Other scholars (e.g. Creath 1996) have already noted that Neurath’s and Carnap’s unity of science theses do not claim to provide a descriptive account of science as it is currently practiced.  In fact, Carnap explicitly agrees with Suppes’ position in §41 of the Aufbau.   Carnap, in his most extended defense of the unity of science thesis (1932/1934), argues only that the various languages of science could, in principle, be connected, not that they are connected.  In sum, the logical positivists’ unity of science thesis asserts that there exists a language in which all (scientific) knowledge can be couched, but not that this language is actually used, on a day-to-day basis, by scientists.    

3. The Elimination of Metaphysics
	The logical empiricists are (in)famous for assuming an anti-metaphysical stance.  All the major figures in the group, as well as most of their patron saints, railed against metaphysics.  But how exactly did the logical empiricists purport to identify and excise the offending terms and claims, and thereby achieve their anti-metaphysical goal? This question becomes especially pressing if one agrees with Michael Friedman’s assertion that “metaphysical neutrality rather than radical empiricism... is... the essence of Carnap’s position” (1999, 110).  Alan Richardson also puts this point strongly: “if there is one defining feature of Carnap’s philosophy, it is the claim that both science and philosophy can be done in a way that is neutral with respect to the traditional issues of metaphysics” (1992, 45).  Such claims need not be restricted to Carnap alone; metaphysical neutrality was a major, if not essential, goal for virtually all central logical positivists.  
How, then, do the logical empiricists expunge metaphysics from science?  The stereotypical view is that the logical empiricists eliminate metaphysics via a comprehensive application of the verificationist criterion of meaning.  (This view has been discounted somewhat in (Richardson 1992, 59) and, more indirectly, in (Creath 1982).)  As I hope to make clear, the verificationist criterion of meaning does play some role in some logical empiricist rejections of metaphysics—however, its role is often a subsidiary one, and exclusive focus upon it leads to a fundamentally incomplete and therefore distorted image of the logical empiricists’ attack on metaphysics.  A more complete picture of the logical empiricists’ anti-metaphysical project requires keeping their unity of science thesis in view.  Roughly put, one criterion separating meaningless metaphysics from cognitively significant science that holds over decades for many logical empiricists is the following:
(M)  An apparently declarative sentence or apparently descriptive term is metaphysical if and only if that (purported) sentence or term cannot be incorporated into a total language of science. 
Furthermore, for the logical empiricists, failures of incorporation into unified science often come in two varieties: a metaphysical claim is either (i.) ungrammatical, or (ii.) grammatical, but isolated.  Case (ii.) arises when a grammatical sentence contains metaphysical terms. 
I must stress that (M) is an idealization.  No formulation of its brevity can accurately characterize the logical empiricists’ views on metaphysics and unity of science, for the historical situation is fairly complex.  Different logical empiricists hold somewhat different views, and a single thinker’s ideas about metaphysics often shift over time.  Furthermore, the biconditional (M) usually does not appear in the texts as such.  Rather, a given logical empiricist virtually always uses only one direction of implication at a time, even though that thinker is committed to both directions, and might even use the other direction elsewhere in the very same work.   So, (M) should be understood as a slogan, from which actual formulations deviate to a greater or lesser degree.  
The next task, then, is to present a more complete and detailed account of the logical empiricists’ rejection of metaphysics across several texts.  By examining the several ‘variants’ of (M), we can determine to what extent (M) captures a basic element of logical empiricist thought, and also what complex, historical nuances (M) elides.  In what follows, I focus on Carnap, for he, more than any other logical empiricist, works out a detailed position on both the unity of science and the rejection of metaphysics.  I then show that Neurath’s texts support attributing (M) to him as well, though his expression of the rejection of metaphysics lacks the fine-grained particulars of Carnap’s. 
Let us begin with Carnap’s treatment of metaphysics in the Aufbau.  How does Carnap there identify metaphysics?  Carnap discusses the concepts of essence, reality, and the mind-body connection (among others), and concludes that each, if taken in their customary sense, is metaphysical.  Each of these purported concepts is deemed metaphysical on the grounds that it cannot be incorporated into any ‘construction system’ [Konstitutionsystem] of the sorts Carnap describes in the Aufbau.  We can phrase Carnap’s criterion for metaphysics in the Aufbau as follows:

(MAufbau) An apparent sentence is metaphysical if and only if it contains concepts that cannot be constructed in a constructional system.
This connection between non-constructability and metaphysics is clear in Carnap’s treatment of the metaphysical ‘problem of reality’:

The concept of reality (in the sense of independence from cognizing consciousness) does not belong within (rational) science, but within metaphysics.  This is now to be demonstrated.  For this purpose, we investigate whether this concept can be constructed, i.e., whether it can be expressed through objects of the most important types which we have already considered, namely, the autopsychological, the physical, the heteropsychological, and the cultural. (1928/1963, 282)

To show that a concept is metaphysical, it must be shown that that concept cannot be constructed from any basic objects—not just phenomenal, ‘autopsychological’ one Carnap develops most fully in the Aufbau.  The mind-body problem (in Carnap’s terms, the ‘parallelism’ between mental states and brain states) is similarly unconstructable:
The question for an explanation of these findings [that mental states and brain states can be placed in a one-to-one correspondence] lies outside the range of science; this already shows itself in the fact that this question cannot be expressed in concepts that can be constructed;… (This holds for any such constructional system and not only for a constructional system of our specific kind.) Rather, the quest for an explanation of that parallelism belongs within metaphysics.  (Carnap 1928/1967, 270-1)
 
The above parenthetical remark shows clearly that, for Carnap, constructability is a more fundamental criterion than verifiability in determining whether a concept or claim is metaphysical, for presumably the ‘specific kind’ to which Carnap refers is the constructional system with autopsychological basis.  Other metaphysical concepts are shown to have the same property; none can be incorporated into a constructional system.  This non-conceptual nature of metaphysics is directly connected to the logical empiricists’ well-known rejection of intuition. Carnap writes: “metaphysics does not wish to grasp its object by proceeding via concepts… but immediately through intuition” (1928/1967, 295), which is ineffable.  The ineffability of so-called metaphysical knowledge was stressed by Schlick (1926).  There are two significant differences between (MAufbau) and (M): first, in the Aufbau, Carnap thinks primarily in terms of concepts; sentences are secondary.  Second, the Aufbau lacks the claim that many sentences of metaphysics are ungrammatical.  This idea, drawn from Wittgenstein’s Tractatus, does not come into prominence in Carnap’s writings until after the Wienerkreis reads the Tractatus intensively together. 
Carnap’s most focused attack on metaphysics is “Overcoming Metaphysics through the Logical Analysis of Language” (1932/1959).  Here Carnap clearly draws the distinction, described above, between the two kinds of pseudo-sentences that cannot be incorporated into the language of science: (i.) ungrammatical strings of symbols, and (ii.) grammatical ‘sentences’ whose terms cannot be connected to the meaningful terms and sentences of the language.  I shall deal with each in turn.  Carnap begins “Overcoming Metaphysics” by noting that there have been several attempts throughout the centuries to abolish metaphysics from the intellectual landscape.  However, “only” with the “development of modern logic” can “the decisive step be taken” in this pursuit (1932/1959, 61).  Why?  A sentence (even if it contains only meaningful words) is meaningless, i.e. metaphysical, if it cannot be expressed in a predicate calculus such as Principia Mathematica.  This is why the ‘development of modern logic’ is so important to the elimination of metaphysics: we pick out metaphysical sentences by finding the strings of symbols which appear meaningful, but cannot be expressed in the logical language of the Principia Mathematica.  This conception of metaphysics is fundamentally Tractarian: whatever cannot be expressed grammatically in the ideal symbolic language of the Tractatus is meaningless metaphysics.  Carnap and Neurath explicitly state that their view on the elimination of metaphysics in the early 1930’s “was in essentials that of Wittgenstein” (Carnap 1934/1937, 322; see also Neurath 1983, 54).
So much for Carnap’s account of metaphysical sentences; when is a term metaphysical, i.e., meaningless?  Carnap takes us on a brief detour through sentences, for a term is shown to be meaningless by showing that atomic sentences containing that term are meaningless.  He asserts that the question “What is the meaning of [an atomic sentence] S?” is equivalent to each of the following two questions:
(1.) What sentences is S deducible from, and what sentences are deducible from S?
(2.) Under what conditions is S supposed to be true, and under what conditions false?
(1932/1959, 62) 
Here again, we see a version of (M).  Here, a sentence (and thereby its constituents) is shown to be meaningful by placing it within a larger ‘inferential network’ ((1.) captures the syntactic aspect of the network, (2.) the semantic).  Grammatical strings that cannot be so placed, Carnap maintains, contain metaphysical terms.  This is very similar to the unconstructable concepts of the Aufbau.  But, one may wonder, what guarantees that any sentences in the larger inferential network are meaningful?—Couldn’t we construct a network of nonsense words? 
Here is where verificationism rears its head.  Carnap states that ‘What is the meaning of S?’, and hence questions (1.) and (2.) above, are also equivalent to “(3.) How is S to be verified?” (ibid.).  For Carnap in 1932, this question is answered by specifying the deducibility relations between S and the “ ‘observation sentences’ or ‘protocol sentences.’ It is through this reduction that the word acquires its meaning” (1932/1959, 63).  However, the specific nature of the protocol sentences is, for Carnap, irrelevant to the elimination of metaphysics: “For our purposes we may ignore entirely the question concerning the content and form of the primary sentences (protocol sentences)”: they could deal with “the simplest qualities of sense” (Mach), “total experience and similarities between them” (the Aufbau), or simply “things” (ibid.).  Furthermore, two years later, Carnap states that which sentences are protocol sentences is a matter of decision (1934/1987).
Carnap’s basic idea, I take it, is that there are some sentences—the ‘primary’ or ‘protocol’ sentences—whose meaningfulness is uncontroversial, assumed, or somehow otherwise guaranteed.  For Carnap has just said that an arbitrary sentence S, and hence the terms it contains, are shown to be meaningful by showing that S is non-trivially inferentially related to some other set of sentences.  Carnap must be assuming that this other set of sentences is meaningful, and that its meaningfulness ‘filters down,’ via implications, to the given sentence S.  We could call this doctrine ‘semantic foundationalism’: just as an epistemic foundationalist holds that there are ‘unjustified justifiers’ which function as the ultimate source for all claims’ justification, a semantic foundationalist holds that some set of sentences functions as the ultimate source for all sentences’ meaning, though their own meaningfulness lacks any further justification from other vocabulary (though they may be justified by practice).  
Carnap’s characterization of the metaphysical does not remain completely static.  In 1934, Logical Syntax of Language appears, and with it a slightly modified program for eliminating metaphysics.  We find the same basic ideas as in “Overcoming Metaphysics,” but with an added wrinkle: the principle of tolerance. 

The view here presented [in accordance with the principle of tolerance] allows great freedom in the introduction of new primitive concepts and new primitive sentences in the language of physics or the language of science in general; yet at the same time it retains the possibility of differentiating pseudo-concepts and pseudo-sentences from real scientific concepts and sentences, and thus of eliminating the former.  [This elimination, however, is not so simple as it appeared to be on the basis of the earlier position of the Vienna Circle… On that view it was a question of “the language” in an absolute sense; it was thought possible to reject both concepts and sentences if they did not fit into the language.] (1934/1937, 322)
That is, we can still avoid metaphysical pseudo-concepts and pseudo-sentences, even if we adopt the Principle of Tolerance and thereby reject the notion that there is a single ‘correct’ language.  As in “Overcoming Metaphysics,” the ‘sentences’ that are ungrammatical, and those apparently descriptive sentences that cannot be connected with the language of empirical science are dismissed as pseudo-sentences, as metaphysics (ibid.).  This is essentially no different from the view advocated two years previous.
Thus Carnap allows in Logical Syntax that there can be many languages of science, but that metaphysical pseudo-sentences can nonetheless be eliminated.  Furthermore, these languages can be fundamentally different from one another, in the sense that they cannot be intertranslated into one another without loss of content.  Two languages will be fundamentally different in this sense whenever one is weaker than another.  For a Carnapian example, consider Languages I and II of Logical Syntax: Language I, intended to capture formally the mathematical intuitionist’s viewpoint, is weaker than Language II, which is expressively rich enough to capture all of classical analysis.  Thus, there are sentences that are grammatical in II, but in I are ungrammatical, and hence metaphysical.  (For example, a sentence about ‘unconstructable numbers’ would count as legitimate in II but would be a metaphysical pseudo-sentence in I.) Similarly, first-order logic is essentially weaker than higher-order logic, so certain sentences condoned in second-order logic would be metaphysical in first-order logic.  We are led to a relativity of metaphysics to languages, codified as follows:

(MLSL):  An apparently declarative sentence or apparently descriptive term is metaphysical with respect to language L if and only if that (purported) sentence or term cannot be incorporated into L.
where ‘incorporation’ is understood as before. 
Does Carnap’s indexing of metaphysics to a particular language threaten my thesis that, for the logical empiricists, metaphysics is to be identified with those propositions that cannot be incorporated into a unified language of science?  I do not think so; rather, my account needs two additional elements of sophistication.  First, the full formulation of the thesis needs to be expanded so that it takes into account the ‘language relativity’ endorsed in Logical Syntax: p is metaphysical with respect to a unified language of science iff p cannot be incorporated into a unified language of science.  Second, I conjecture that the usual or standard extension of the term ‘metaphysics’ simpliciter (i.e., without the index to a language), is probably well captured by taking the arbitrary language L in (MLSL) to be a unified language of science.  
Lastly, perhaps this relativization of metaphysics to languages reveals something insightful about the way the term ‘metaphysics’ is used.  Intuitionists do find something suspect about the unconstructable numbers of classical mathematics, and some would be inclined to call claims about such entities ‘metaphysics.’  Heyting, expressing the intuitionist viewpoint, writes: “If ‘to exist’ does not mean ‘to be constructed,’ it must have some metaphysical meaning” (1971/1983, 67).  Similarly, philosophers who find second-order logic suspicious call its quantification over predicates ‘Platonism,’ after the grandfather of all metaphysicians.  And if we think of metaphysics as nonsense, as the Vienna Circle and Wittgenstein do, then it becomes clearer that the label of ‘metaphysical’ should be indexed to a particular language—for what is meaningful in one language simply will not be in another.  
	In 1950’s “Empiricism, Semantics, and Ontology,” Carnap’s basic idea for identifying metaphysics is essentially the same.  However, the terminology has shifted: instead of speaking of constructional systems or languages, Carnap now speaks of linguistic frameworks.  But here again, a claim is shown to be non-metaphysical by incorporating it into a (pragmatically) acceptable linguistic framework.

[T]he concept of reality…in internal questions is… [a] scientific, non-metaphysical concept.  To recognize something as a real thing or event means to succeed in incorporating it into the system of things…, according to the rules of the framework. … (1950/1956, 207; my italics) 

The importance of a shared scientific language for identifying metaphysics also recurs here.  It is on precisely these grounds that Carnap criticizes philosophers who ask the ‘external’ question “Are there numbers?”:
Unfortunately, these philosophers have not given a formulations of their question in the common scientific language.  Therefore… they have not succeeded in giving the external question cognitive content. (1950/1956, 209)

And questions without ‘cognitive content’ are metaphysical.  Thus, Carnap’s attitude towards metaphysics in 1950 is very closely related to his view in the twenties; linguistic frameworks replace construction systems, but the basic strategy for identifying and eliminating metaphysics remains the same.
	So much for Carnap’s views on metaphysics; what of Neurath?  Though he eschews Carnap’s formal, precise languages in favor of his ‘universal jargon’ based on everyday language, he shares the fundamental idea found in Carnap: an apparently meaningful sentence or term is metaphysical if and only if it cannot be incorporated into unified science.  First, the ‘only if’ direction:
If it [a proposed scientific sentence] is… meaningless—i.e., metaphysical —then of course it falls outside the sphere of unified science. (1983, 58)

And the ‘if’ direction:

statements that through their structure or special grammar could not be placed within the language of the encyclopedia—in general ‘isolated’ statements, … are statements ‘without meaning in a certain language’.  For these statements the Vienna Circle has often used the term ‘metaphysical statements’. (1983, 161)

Note that Neurath mentions the strictures against both ungrammatical and isolated ‘sentences.’  Where Carnap employs a constitution system or a linguistic framework, Neurath uses an encyclopedic language based on everyday communication instead; but otherwise, their views are very close.
Recall the notion of ‘semantic foundationalism,’ mentioned above: a sentence’s meaningfulness is demonstrated by showing that that sentence is connected via consequence-relations to sentences whose meaningfulness is given antecedently.  The question Carnap was undecided on in 1932 was ‘Which sentences are given as meaningful?’  Neurath’s version of unified science presents us with another way to answer this question.  Neurath repeatedly states that unified science should begin from everyday language, with minor corrections.  Why?  One possible reason is that everyday language is meaningful if any language is; everyday language would be the most indisputable case of a meaningful language.  We are more committed to the meaningfulness of everyday language than any other.  Thus, if we have to pick a ‘semantic foundation,’ everyday language seems the best we can do.  (There are other reasons Neurath starts with everyday language: he values the democratization and popularization of scientific knowledge, and he is suspicious of any framework that aims to break loose of our present historical situation.) 
One might criticize my interpretation of Neurath’s claims about the unity of science as follows: a central aim of work in unified science is demolition of the barriers between the scientific study of nature and of the mind; my interpretation misses that aspect entirely.  I concede, of course, that Neurath repeatedly and unequivocally urged the value of breaking down disciplinary barriers.  But, interestingly, Neurath claims that the motivation underlying the separation of the sciences is metaphysical.  When his program is realized,
each basic decomposition of unified science is eliminated…, for example, that into ‘natural sciences’ and ‘mental sciences’… .  The tenets with which we want to justify the division are… always of a metaphysical kind, that is, meaningless. (1983, 68)
So, according to Neurath, any assertion used to justify a strict division of the sciences is metaphysical.  If the various sciences were unified, then any such assertion would be ruled out.  Thus, unified science, which shows disciplinary barriers are not insuperable, eliminates a certain kind of metaphysics—specifically, it eliminates any theory that purports to deal with “a special sphere of the ‘soul’” (1983, 73), distinct from the remainder of the spatiotemporal world.  Unification of the sciences may be valuable for its own sake, but it also serves to eliminate metaphysics. 
	
4. Conclusion
	Thus far, I have argued that, in the writings of central logical empiricists, there is a close conceptual connection between the unity of science thesis and the elimination of metaphysics, and that this connection is captured, to a first approximation, by (M).  In closing, I present one piece of evidence that this connection is not merely conceptual, but also genealogical.  That is, the term ‘unified science’ [Einheitswissenschaft], suggested by Neurath, sprung directly out of the Vienna Circle’s elimination of metaphysics.  Neurath, recalling the Circle’s discussion of the Tractatus, explains how he came to introduce the term. 

Eliminating ‘meaningless’ sentences became a kind of game… But I very soon felt uneasy, when members of our Vienna Circle suggested that we should drop the term ‘philosophy’ as a name for a set of sentences … but use it as a name for the activity engaged in improving given sentences by ‘demetaphysicalizing’ them  … Thus I came to suggest as our object, the collection of material, which we could accept within the framework of scientific language; for this I thought the not-much-used term ‘Unified Science’ (Einheitswissenschaft…) a suitable one. (1983, 231)
Thus, the very term ‘unified science’ arose directly from a desire to re-name the anti-metaphysical goal of the Wienerkreis.  

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