## 1. Bridgman's ideas on operational analysis


### 1.1 Background and motivation



Percy Williams Bridgman (1882-1961) was a physicist at Harvard
University whose pioneering work in the physics of high pressures was
rewarded with a Nobel Prize in
 1946.[2]
 His chief scientific contribution was made possible by technical
prowess: in his laboratory Bridgman created pressures nearly 100 times
higher than anyone else had achieved before him, and investigated the
novel behavior of various materials under such high pressures. But
Bridgman was placed in a predicament by his own achievements: at such
extreme pressures, all previously known pressure gauges broke down;
how was he even to know what levels of pressure he had in fact
reached? (see Kemble, Birch and Holton 1970) As he kept breaking his
own pressure records, Bridgman had to establish a succession of new
measures fit for higher and higher pressures. Therefore it is no
surprise that he thought seriously about the groundlessness of
concepts where no methods were available for their measurement.



Another important stimulus to his philosophical thinking was his
encounter with the revolutionary new physics of the early 20th
century. Bridgman's concerns about the definition and meaning of
scientific concepts were forged in the general climate of shock
suffered by physicists at that time from a barrage of phenomena and
theoretical ideas that were entirely alien to everyday expectations,
culminating with quantum mechanics and its "Copenhagen"
interpretation. In a popular article, Bridgman wrote: "if we
sufficiently extend our range we shall find that nature is
intrinsically and in its elements neither understandable nor subject
to law" (1929, 444).



Especially important for Bridgman's thinking was Albert
Einstein's special theory of relativity. Bridgman credited an
unexpected teaching assignment in 1914 for his first real encounter
with special relativity, which gave him considerable distress as he
tried to clarify the confusing conceptual situation surrounding the
theory (Bridgman in Frank 1956, 76). At the heart of special
relativity was Einstein's recognition that judging the
simultaneity of two events separated in space required a different
operation from that required for judging the simultaneity of two
events happening at the same place. Fixing the latter operation was
not sufficient to fix the former, so a further convention was
necessary, which Einstein supplied in the form of his operation of
sending light beams from each of the events in question to the
midpoint between their locations, to see if they arrive there at the
same time. How superior this way of thinking was, compared to Isaac
Newton's declaration that he would "not define Time,
Space, Place or Motion, as being well known to all" (quoted in
Bridgman 1927, 4)! Bridgman felt that all physicists, including
himself, had been guilty of unthinking uses of concepts, especially on
the theoretical side of physics.



Bridgman's sentiment arising out of these reflections, however,
was not the familiar one of happy celebration of Einstein's
genius. He rather regretted the sorry state of physics which had
necessitated Einstein's revolution. Einstein showed what
dangerous traps we could fall into by stepping into new domains with
old concepts in an unreflective way. Anyone thinking in operational
terms would have recognized from the start that the meaning of
"distant simultaneity" was not fixed unless an operation
for judging it was specified (Bridgman 1927, 10-16). In
Bridgman's view, Einstein's revolution would never have
been necessary, if classical physicists had paid operational attention
to what they were doing. He thought that any future toppling of
unsound structures would become unnecessary if the operational way of
thinking could quietly prevent such unsound structures in the first
place. Operational awareness was required if physics was not to be
caught off-guard again as it had been in 1905: "We must remain
aware of these joints in our conceptual structure if we hope to render
unnecessary the services of the unborn Einsteins" (Bridgman
1927, 24).



Bridgman's impulse was to specify every possible detail of his
operations, because any detail *could* make an important
difference. Note the following passage, inspired by the shock of
learning from the special theory of relativity that the measured
length of an object was not independent of its velocity:



> 
> Now suppose we have to measure a moving street car. The simplest, and
> what we may call the "naive" procedure, is to board
> the car with our meter stick and repeat the operations we would apply
> to a stationary body. Notice that this procedure reduces to that
> already adopted in the limiting case when the velocity of the street
> car vanishes. But here there may be new questions of detail. *How
> shall we jump on to the car with our stick in hand?* Shall we run
> and jump on from behind, or shall we let it pick us up in front? Or
> perhaps does now the material of which the stick is composed make a
> difference, although previously it did not? All these questions must
> be answered by experiment. (Bridgman 1927, 11; emphasis added)
> 


### 1.2 Length as the showcase of operational analysis



Bridgman found that the challenges of the unknown were amply present
even in very prosaic circumstances. Therefore he chose to open his
discussion of operational analysis in *The Logic of Modern
Physics* (Bridgman 1927) with the example of the most mundane of
all scientific concepts: length. He was both fascinated and troubled
by the fact that "essential physical limitations" forced
scientists to use different measurement operations for the same
concept in different domains of phenomena. Length is measured with a
ruler only when we are dealing with dimensions that are comparable to
our human bodies, and when the objects of measurement are moving
slowly relative to the measurer. To measure, say, the distance to the
moon, we need to infer it from the amount of time that light takes to
travel that distance and return, and that is also the procedure taken
up in Einstein's theorizing in special relativity; "the
space of astronomy is not a physical space of meter sticks, but is a
space of light waves" (Bridgman 1927, 67). For even larger
distances we use the unit of "light-year," but we cannot
actually use the operation of sending off a light beam to a distant
speck of light in the sky and waiting for years on end until hopefully
a reflected signal comes back to us (or our descendants). Much more
complex reasoning and operations are required for measuring any
distances beyond the solar system:



> 
> Thus at greater and greater distances not only does experimental
> accuracy become less, but the very nature of the operations by which
> length is to be determined becomes indefinite.... To say that a
> certain star is 105 light years distant is actually and
> conceptually an entire different *kind* of thing from saying
> that a certain goal post is 100 meters distant. (Bridgman 1927,
> 17-18; emphasis original)
> 



Thus operational analysis reveals that length is not one homogeneous
concept that applies straightforwardly in the whole range of cases in
which we use it:



> 
> In *principle* the operations by which length is measured
> should be *uniquely* specified. If we have more than one set of
> operations, we have more than one concept, and strictly there should
> be a separate name to correspond to each different set of operations.
> (Bridgman 1927, 10; emphases original)
> 



In practice scientists do not recognize multiple concepts of length,
and Bridgman was willing to concede that it was allowable to use the
same name to represent a series of concepts, if the different
measurement operations gave mutually consistent numerical results in
the areas of overlap:



> 
> If we deal with phenomena outside the domain in which we originally
> defined our concepts, we may find physical hindrances to performing
> the operations of the original definition, so that the original
> operations have to be replaced by others. These new operations are, of
> course, to be so chosen that they give, within experimental error, the
> same numerical results in the domain in which the two sets of
> operations may be both applied. (Bridgman 1927, 23)
> 



However, such numerical convergence between the results of two
different operations was regarded by Bridgman as merely a
"practical justification for retaining the same name" for
what the two operations measured (Bridgman 1927, 16).



Even in such convergent situations, we have to be wary of the danger
of slipping into conceptual confusion through the use of the same word
to refer to the subjects of different operations. If we do not temper
our thoughts with the operationalist conscience always referring us
back to concrete measurement operations, we may get into the sloppy
habit of using one word for all sorts of different situations (without
even checking for the required convergence in the overlapping
domains). Bridgman warned: "our verbal machinery has no built-in
cutoff" (1959a, 75). Similarly we could be misled by the
representation of a concept as a number into thinking that there is
naturally an infinitely extendable scale for that concept, since the
real-number line continues on to infinity. It would also be easy to
think that physical quantities must meaningfully exist down to
infinite precision, just because the numerical scale we have pinned on
them is infinitely divisible. Bridgman issued a stark reminder:



> 
> Mathematics does not recognize that as the physical range increases,
> the fundamental concepts become hazy, and eventually cease entirely to
> have physical meaning, and therefore must be replaced by other
> concepts which are operationally quite different. For instance, the
> equations of motion make no distinction between the motion of a star
> into our galaxy from external space, and the motion of an electron
> about the nucleus, although physically the meaning in terms of
> operations of the quantities in the equations is entirely different in
> the two cases. The structure of our mathematics is such that we are
> almost forced, whether we want to or not, to talk about the inside of
> an electron, although physically we cannot assign any meaning to such
> statements. (Bridgman 1927, 63)
> 



Bridgman thus emphasized that our concepts did not automatically
extend beyond the domains in which they were originally defined. He
warned that concepts in far-out domains could easily become
meaningless for lack of applicable measurement operations. The case of
length in the very small scale makes that danger clear. Beyond the
resolution of the eye, the ruler has to be given up in favor of
various micrometers and microscopes. When we get to the realm of atoms
and elementary particles, it is not clear what operations could be
used to measure length, and not even clear what "length"
means any more.


### 1.3 Critique of other physical concepts



After introducing operational analysis with that refreshing discussion
of the length concept, Bridgman published a long series of critical
re-assessments of various fundamental physical concepts. His
ruminations on length were extended into general commentary on the
nature of space, and the concept of time received a similar treatment.
His views on space and time were reminiscent of Henri
Poincare's and Pierre Duhem's: Bridgman noted that
clocks had to be used in the empirical determination of the basic laws
of physics, while our confidence that a clock ticks regularly was
founded in the basic laws of physics governing its mechanism. Suppose
we are trying to test the general theory of relativity by measuring
the red-shift of light coming out of a heavy body:



> 
> If the vibrating atom is a clock, then the light of the sun is shifted
> toward the infra-red, but how do we know that the atom is a clock
> (some say yes, others no)? If we find the displacement physically have
> we thereby proved that general relativity is physically true, or have
> we proved that the atom is a clock, or have we merely proved that
> there is a particular kind of connection between the atom and the rest
> of nature, leaving the possibility open that neither is the atom a
> clock nor general relativity true? (Bridgman 1927, 72-73)
> 



Bridgman found these reflections liberating as well as troubling.
Basic space-time concepts are not uniquely determined *a
priori*. For example, he noted that the notion of velocity common
to classical mechanics and special relativity was not the only one in
line with our intuitions of what velocity meant. Consider this
alternative: "a traveler in an automobile measures his velocity
by observing the clock on his instrument board and the mile stones
which he passes on the road." If we adopt such a procedure we
will find that the speed of light is infinite, if special relativity
is correct about time dilation: with the car going at the speed of
light according to an observer standing on the road, the clock on the
car will not advance at all while the car passes any number of
milestones. This alternative concept of velocity would have the
advantage that "there would be no limit to the velocity which
can be imparted to material bodies on giving them unlimited
energy", which seems intuitively "natural and
simple". But assigning an infinite velocity to light is also
"most unnatural, particularly if we favor a medium point of
view." So there was a dilemma: "all sorts of phenomena
cannot at the same time be treated simply." (Bridgman 1927,
98-100)



In the latter parts of *The Logic of Modern Physics* Bridgman
gave a fascinating array of discussions on the concepts of force,
mass, energy, light and field, and the theories of thermodynamics,
relativity and quantum mechanics. These thoughts were developed
further in the remaining decades of his life, and collected in
subsequent volumes including *The Nature of Physical Theory*
(1936), *The Nature of Thermodynamics* (1941), *Reflections
of a Physicist* (1950, second edition 1955), *The Nature of
Some of Our Physical Concepts* (1952), and *The Way Things
Are* (1959a). Bridgman made a searching examination of the
familiar concepts of classical physics, checking to see if they
retained operational meaning in domains of phenomena that were
unfamiliar to the creators of classical physics. In some cases his
analysis showed that the classical concepts were operationally unsound
even in the contexts in which they were originally created. Later in
life he stated that his initial foray into philosophy was motivated by
his "disquietude" about physics, especially
electrodynamics and thermodynamics, in which "the fundamental
understanding of even the acknowledged leaders in physics were
inadequate" (Bridgman 1959b, 519). In contrast, he thought that
on the whole the contemporary development of quantum theory went in
the right direction, especially in Werner Heisenberg's version
of it, which discarded classical concepts where they did not apply
(e.g., space-time orbits for electrons), and crafted new concepts with
clear operational meaning in new domains of phenomena. However, he was
not entirely satisfied with Niels Bohr's doctrine that all the
operations of physics needed to be explained in the "macroscopic
language of daily life or of present-day philosophy"; rather, he
thought we needed to develop a "more adequate macroscopic
language" (1959b, 526).



Interestingly, Bridgman never stopped thinking about relativity. The
operationalist lesson he had taken from Einstein was so dear to him
that he did not shrink from criticizing Einstein himself when the
latter seemed to betray his own principles in the general theory of
relativity. Already in *The Logic of Modern Physics* he had
opined: "I personally question whether the elements of
Einstein's formulation, such as curvature of space-time, are
closely enough connected with immediate physical experience ever to be
accepted as an ultimate in a scheme of explanation, and I very much
feel the need for a formulation in more intimate physical terms"
(1927, 176). Years later, when Bridgman was invited by P. A. Schilpp
to contribute to the Library of Living Philosophers volume on
Einstein, he issued the following "indictment" against
Einstein: "he has carried into general relativity theory
precisely that uncritical, pre-Einsteinian point of view which he has
so convincingly shown us, in his special theory, conceals the
possibility of disaster" (Bridgman 1949a, 354; reprinted in
Bridgman 1955, 337). Einstein brushed aside Bridgman's
objection, merely stating that for a formal system to qualify as a
physical theory it was "not necessary to demand that all of its
assertions can be independently interpreted and 'tested'
'operationally'" (Einstein in Schilpp 1949, 679).
This exchange is reminiscent of how Einstein responded with bemused
incomprehension to Heisenberg's protest that he was following
Einstein's own lesson in treating only directly observable
quantities in his matrix mechanics (Heisenberg 1971, 62-69).
Bridgman also carried out further operational analysis of special
relativity, and his late thoughts on the subject were published
posthumously in *A Sophisticate's Primer on Relativity*
(1962).



Bridgman's critique of concepts in physics also led naturally to
a philosophical critique of some general conceptions underlying
physics, such as simplicity, atomism, causality, determinism, and
probability. He also gave critical considerations to mathematics and
its application to the physical world. There was no stone Bridgman was
willing to leave unturned in his relentless critique. He went as far
as to declare: "arithmetic, so far as it purports to deal with
actual physical objects, is also affected with the same penumbra of
uncertainty as all other empirical science" (Bridgman 1927,
34).



Among physicists Bridgman's reflections found a strong echo,
especially in the early days. Perhaps that was natural: it has been
said, by Bridgman himself too, that operationalism arose from
observing "physicists in action" (Bridgman in Frank 1956,
80). Gerald Holton (1995a, 224) recalls what an "electrifying
experience" it was for himself and many other physicists to read
*The Logic of Modern Physics* for the first time, but explains
that the "immense power" of Bridgman's work is
"not that the work brings to the reader a message never thought
of before, but that it lays open, with clarity, what the reader has
been trying to formulate on his or her own". In Mary
Hesse's view Bridgman only gave "the most explicit
formulation" of a common view, which was shared even by Arthur
Eddington, whose metaphysical musings about physics Bridgman took a
strong exception to: "A physical quantity is defined by the
series of operations and calculations of which it is the result"
(quoted in Hesse 1952, 218).



Bridgman's philosophical views, however, were not just an
articulation of views widely shared by physicists. Albert E. Moyer
(1991) stresses that Bridgman's operational analysis served the
function of making special relativity and quantum mechanics more
palatable by showing how they could be understood in a way that most
physicists could cope with. But the operational way of rendering the
new physics was not agreeable to all physicists (or philosophers of
physics), although it was consonant with the stances taken by some
leaders in the field including Bohr, Heisenberg and Oppenheimer (Moyer
1991, 376, 389). We have already seen Einstein's objections to
Bridgman's interpretation of relativity. Mario Bunge (1988)
argues that operationalism is "written into the standard (or
Copenhagen) interpretation of the quantum theory", but that this
interpretation gives a clear misrepresentation of the actual practices
in quantum physics. Analyzing Dirac's version of the theory,
Hesse (1952) concluded that concepts of quantum theory did not have
simple operational meanings, and Dirac's
"observables" were concepts without operational
definitions. Rather, in Hesse's view, concepts in such
situations only derived their meanings from theoretical analogies.


### 1.4 Implications outside physics



Bridgman also extended his operationalist thinking by considering its
implications outside physics. This was important to him at least from
the time of *The Logic of Modern Physics*, in which he
ventured: "many of the questions asked about social and
philosophical subjects will be found to be meaningless when examined
from the point of operations. It would doubtless conduce greatly to
clarity of thought if the operational mode of thinking were adopted in
all fields of inquiry as well as in the physical" (30-32).
To Bridgman it was clear that "to adopt the operational point of
view ... means a far-reaching change in all our habits of
thought". He knew that in practice this would be a very
difficult thing to do: "Operational thinking will at first prove
to be an unsocial virtue; one will find oneself perpetually unable to
understand the simplest conversation of one's friends, and will
make oneself universally unpopular by demanding the meaning of
apparently the simplest terms of every argument". Perhaps this
throwaway remark was a sign of things to come, as Bridgman would in
the end find himself rather isolated and plagued by misunderstanding,
even among those who found his philosophical ideas worth discussing,
as we will see in Section 2.



Bridgman did not develop in detail his operationalist ideas in
relation to any other science than physics, apparently content to
leave that job to the specialists in the respective fields. Some other
scientists and philosophers did take up Bridgman's call for
operationalist reforms, with interesting consequences. Now, it would
be fair to say that operationalism did not change the practice of
physics itself drastically from what it would have been anyway, and
most physicists followed him only as far as he asserted what was
common sense to them. The situation was perhaps similar in other
physical sciences, too. For example, operational thinking has always
been an important part of chemistry, sometimes explicitly so, for
example in Lavoisier's famous "operational"
definition of a chemical element as a substance that has not yet been
decomposed further. David Hull (1968) indicates that some key terms in
biology has been criticised for not being operational, and calls for a
nuanced view on the matter. Hugh Petrie (1971) laments that a
positivist dogma originating from a misunderstanding of operationalism
has had a negative influence on the social sciences. Moyer (1991,
393-394) records Bridgman's influence on the University of
Chicago economist Henry Schultz, and the strong and explicit
operationalist orientation of Paul Samuelson's early work. Paul
Marshall (1979) argues, with particular attention to political
science, that operationalism has left a methodological legacy in the
social sciences lasting well beyond its decline within the philosophy
of science.



The field that was most significantly and expressly affected by
operationalism was psychology. Behaviorist psychologists took up
operationalism (or *operationism*, as it was more often called
in psychology) as a weapon in their fight against more traditional
psychologists, especially those who prized introspection as the most
important source of psychological knowledge. The Harvard psychologist
Edwin Boring (1886-1968) saw Bridgman's philosophy as a
modern substitute for positivism, and he seems to have coined the term
"operationism" (see Walter 1990, 178). It was
Boring's student Stanley Smith Stevens (1906-1973) who was
perhaps the most aggressive promoter of operationism in psychology
(Hardcastle 1994; Feest 2005). Stevens saw operationism as a sure
method of increasing rigor in psychological experiments and discourse,
asserting that "to experience is, for the purpose of science, to
react discriminately", since these reactions are what science
can measure and record publicly (quoted in Feest 2005, 136). Echoing
Bridgman's take on Einstein, Stevens declared in 1935:
"The revolution that will put an end to the possibility of
revolutions is the one that defines a straightforward procedure for
the definition and validation of concepts. ... Such a procedure
is the one which tests the meaning of concepts by appealing to the
concrete operations by which the concept is determined. We may call it
*operationism*" (quoted in Walter 1990, 180).



In his concrete research in psychology Stevens focused on
psychophysics, starting with his Ph.D. dissertation on the perceived
attributes of tones, written under Boring's supervision. Another
notable operationist in psychology was Edward Chace Tolman
(1886-1959), also a Harvard Ph.D., who taught for most of his
life at the University of California at Berkeley. Starting with his
research on problem-solving behavior in rats, Tolman gave an
operational treatment of desire, for example operationalizing hunger
in terms of "time since last feeding". Tolman did not deny
that hunger was a subjective feeling, nor was it the main object of
his research, but insisted that controlling for degrees of hunger in
his experiments required tractable operations that would allow
scientists to get a hold on something related to that subjective
experience (see Feest 2005, 136-138). In Gustav Bergmann's
assessment, operationism helped behaviorism to move from its initial
metaphysical Watsonian variety to its modern version (Bergmann in
Frank 1956, 53).



Despite the strong popularity of operationist behaviorism in certain
quarters, it never commanded a complete consensus even in American
psychology. Perhaps the most unexpected opposition came from Bridgman
himself. Behaviorists were wanting to use operations to attain
objectivity in psychology, which for them meant taking psychological
discourse away from attempts to describe private experience. This was
just the wrong move in Bridgman's view, as we will see in more
detail in Section 2.4 below. Bridgman engaged in some discussion with
Stevens but discovered that the latter's enthusiasm for
"operational ideas" was really for something that he could
not agree with. By 1936 he declared privately: "I have rather
washed my hands of him" (quoted in Walter 1990, 184).
Bridgman's disagreement with B. F. Skinner (1904-1990) was
even more severe, and resulted in a prolonged dispute between the two
(Holton 2005; Walter 1990, 188-192). Operationism became a
subject of great controversy in psychology, epitomized in a 1945
special issue of the *Psychological Review* devoted to a
symposium on operationism suggested by Boring, who remained supportive
of the movement at some critical distance.



Uljana Feest considers that operationism did leave a lasting and
positive legacy in psychology. The maxim of operationism dictates that
psychologists should provide concepts with operational definitions by
specifying paradigmatic experimental conditions for their application,
and this remains a useful methodological tenet (Feest 2011, 403). In
Feest's view a sufficiently sophisticated understanding of
operationism in psychology was only reached by the second generation
of operationists, who consciously employed what she calls the
methodological (rather than positivist) reading of operationism.
Feest's analysis reinforces R. C. Grace's (2001)
view that the early operationist psychologists made the mistake of
adopting a very restrictive version of Bridgman's ideas, but
that this mistake was corrected in the 1950s with the adoption of
"multiple" or "convergent" operationism. These
sophisticated operationists did not take operational definitions to
give full and fixed meanings of concepts, but used them as provisional
tools aiding their scientific investigations. As Feest puts it:
"I believe that in offering operational definitions, scientists
were *partially and temporarily* specifying their usage of
certain concepts by saying which kinds of empirical indicators they
took to be *indicative of* the referents of the
concepts."(Feest 2005, 133; emphases original)


## 2. Critiques of operationalism



Despite the initial popularity of Bridgman's ideas, by the
middle of the 20th century the common reactions among philosophers and
philosophically-minded scientists were strongly critical.
Operationalism received many high-profile debates, among them a
symposium held at the annual meeting of the American Association for
the Advancement of Science (AAAS) in 1953 (published in Frank 1956),
and the *Psychological Review* issue mentioned above. On such
occasions Bridgman attempted to refine and defend his views, but also
found that the debate was moving in directions that both surprised and
disturbed him. In his contribution to the AAAS symposium he
exclaimed:



> 
> There would seem to be no reason why I am better fitted than anyone
> else to open this discussion. As I listened to the papers I felt that
> I have only a historical connection with this thing called
> "operationalism." In short, I feel that I have created a
> Frankenstein, which has certainly got away from me. I abhor the word
> *operationalism* or *operationism*, which seems to imply
> a dogma, or at least a thesis of some kind. The thing I have envisaged
> is too simple to be dignified by so pretentious a name. (Bridgman in
> Frank 1956, 75-76)
> 



Still, arguably it was not Bridgman's own ideas about
operational analysis but the Frankenstein of operationalism that had a
more significant impact in philosophy and science, so this survey of
operationalism must deal with how other people responded to
operationalism as they saw it. In the course of the discussion I will
try to point out some places where there were clear misunderstandings
of Bridgman's ideas, and also other places where Bridgman
himself was ambivalent or ambiguous, rather than simply
misunderstood.



Given the timing and contexts of the publication of Bridgman's
ideas, the philosophical debates surrounding them were to a large
extent framed in relation to logical positivism, which was just making
its big impact on the American philosophical scene. The young Herbert
Feigl (1902-1988), with a personal introduction by no less than
Moritz Schlick, came to Harvard in 1930 with the express purpose of
learning from Bridgman, despite the latter's warning that he did
not have much to teach (Walter 1990, 164-165; Moyer 1991, 391).
Bridgman's insistence on operational meaningfulness had at least
a surface resemblance to the logical positivists' verification
theory of meaning. Bergmann thought Bridgman had given a
"scientist's version" of the latter (Bergmann in
Frank 1956, 55), and Carl Hempel regarded operationalism and logical
positivism as "closely akin" to each other (Hempel in
Frank 1956, 56). And it is not difficult to see how a kindred
philosophical doctrine coming from a world-class scientist would have
captured the logical positivists' imagination.



When subjected to the scrutiny of professional philosophers, however,
Bridgman's ideas were soon exposed as unsystematic and
undeveloped, as he freely admitted himself. Moreover, it became
evident that his ideas did not help logical positivists solve the key
problems that they were struggling with. After the initial
fascination, the standard positivist (and post-positivist) reaction to
operationalism was disappointment, and operationalism was often seen
as a failed philosophy that did not live up to its promises.


### 2.1 Operational definitions do not exhaust meaning



Nowhere was the positivist disappointment with Bridgman sharper than
in considerations of operationalism as a theory of meaning. There was
a set of objections which together amounted to a complaint that
operational definitions did not give a sufficient account of the
meaning of concepts, even where there were operations clearly relevant
to the concepts in question.



The core of the problem here is an overly restrictive notion of
meaning, which reduces it to measurement; I will call this
*Bridgman's reductive doctrine of meaning*. Although
Bridgman was not proposing a general philosophical theory of meaning,
he did make remarks that revealed an impulse to do so. Consider the
following statement, the last part of which I have already quoted:



> 
> We evidently know what we mean by length if we can tell what the
> length of any and every object is, and for the physicist nothing more
> is required. To find the length of an object, we have to perform
> certain physical operations. The concept of length is therefore fixed
> when the operations by which length is measured are fixed: that is,
> the concept of length involves as much as and nothing more than the
> set of operations by which length is determined. In general, we mean
> by any concept nothing more than a set of operations; the concept is
> synonymous with the corresponding set of operations. (Bridgman 1927,
> 5)
> 



Similarly, he also displayed an impulse to use operations to make a
criterion of meaningfulness: "If a specific question has
meaning, it must be possible to find operations by which an answer may
be given to it." (Bridgman 1927, 28)



One lesson we can take from Bridgman's troubles is that meaning
is unruly and promiscuous. The kind of absolute control on the meaning
of scientific concepts that Bridgman wished for is not possible. The
most control that can be achieved is for the scientific community to
agree on an explicit definition and to respect it. But even firm
definitions can only constrain the uses of a concept. The entire world
can agree to define length by the standard meter in Paris (or by the
wavelength of a certain atomic radiation), and that still comes
nowhere near *exhausting* all that we mean by length. Bridgman
himself later specifically admitted that his statement that meanings
were synonymous with operations was "obviously going too far
when taken out of context" (1938, 117). Especially compared with
the notion of "meaning as use," often traced back to the
later phase of Ludwig Wittgenstein's
 work,[3]
 it is easy to recognize the narrowness of Bridgman's initial
ideas. Bridgman's later gloss on his ideas was in fact rather
late-Wittgensteinian: "To know the meaning of a term used by me
it is evident, I think, that I must know the conditions under which I
would use the term" (1938, 116). As measurement operations
provide only one specific context in which a concept is used,
operational definitions can only cover one particular aspect of
meaning.



Recognizing the restrictiveness of Bridgman's early remarks on
meaning gives us a useful framework for understanding one common
objection to operationalism. As Donald Gillies (1972, 6-7)
emphasizes, if we accept the most extreme kind of operationalism,
there is no point in asking whether a measurement method is valid; if
the measurement method defines the concept and there is nothing more
to the meaning of the concept, the measurement method is automatically
valid, as a matter of convention or even tautology. Metrological
validity becomes an interesting question only if the concept possesses
a broader meaning than the specification of the method of its
measurement. Not only Bridgman himself but the second-generation
operationist psychologists recognized this point very clearly, in
their discussions of construct validity (see, for example, Cronbach
and Meehl 1955). The measurement method can be said to be valid if it
coheres with the other aspects of the concept's meaning. That
way we may also make a judgment about whether an operational
definition (or any other kind of definition) is an appropriate one,
depending on how well it coheres with other elements of the
concept's meaning and how beneficially it controls other
elements of meaning.


### 2.2 Operational definitions are not required for all useful concepts



So far I have noted that an operational definition is not sufficient
to express a concept's meaning fully. Going further than that,
many critics of operationalism have argued that not every good
scientific concept needs to have an operational definition. If
operationalism means demanding that every concept and every
inferential step should have an immediate operational significance, it
constitutes an overly restrictive empiricism. At times Bridgman did
appear to be making such a demand, as illustrated in the poignant
episode (discussed in Section 1.3 above) in which he criticized
Einstein for betraying his own operationalist lesson in the general
theory of relativity. Einstein's view was that there was no
reason for physicists to shrink from using a non-operational concept
if that delivered good results.



Einstein was self-consciously opportunistic in his methodological
eclecticism, but philosophers wanted to find a more general rationale
for freeing scientific theorizing from operationalist
micro-management. The crux of the problem here for the operationalist
is that theoretical concepts are much too useful in science. Bridgman
actually acknowledged from early on that there were good theoretical
concepts that were not amenable to direct operationalization,
illustrating the point with the example of stress and strain inside a
solid body (1927, 53-54), and the wavefunction in quantum
mechanics (Bridgman in Frank 1956, 79). Bridgman saw clearly that
these theoretical concepts only had indirect connections with physical
operations, but he did not see any problems with that. He in fact went
so far as to say: "there need be no qualms that the operational
point of view will ever place the slightest restriction on the freedom
of the theoretical physicist to explore the consequences of any free
mental construction" (Bridgman in Frank 1956, 79; see also
Bridgman 1949b, 256). All that was required was that the theoretical
system touched the operational ground somewhere, eventually. However,
in that case Bridgman's message was the same as
Einstein's, as the physicist R. B. Lindsay pointed out (Lindsay
in Frank 1956, 71-72).



Bridgman's position on the matter of theoretical concepts was
complicated, and perhaps not entirely self-consistent (I will return
to this point in Section 3.3). One common objection to operationalism
is based on a misunderstanding that reveals an essential difference
between Bridgman and most of his critics. It is often said that
operationalism cannot be right because each scientific concept can be
measured in various ways. This criticism is based on the presumption
that the concept in question *has* unity, which means that its
definition must also be unified. If there are a variety of measurement
methods all of which apply to one concept, then measurement methods
cannot be what supplies the unified definition; instead, some
theoretical account must be given as to how the variety of operations
under consideration serve to measure the same thing. Bridgman, in
contrast, had no such presumption of conceptual unity. For him, the
initial position to take was that if there are different methods of
measurement we have different concepts, as he said about
"tactual" and "optical" length being two
different concepts. Now, it *could* be that there is one aspect
of reality that different measurement operations all get at, but that
is something to be demonstrated, not to be assumed at the outset. The
possibility of unity can be entertained if the minimum condition of
numerical convergence is met--that is, if two measurement
operations have an overlapping range and their results agree in the
overlap. Still, Bridgman maintained some skepticism about whether it
was safe for us to infer real conceptual unity from such numerical
convergence.



Bridgman's ambivalence about conceptual unity elicited a serious
worry about the systematic import of scientific concepts and theories,
most astutely expressed by Hempel (1966, 91-97).
Bridgman's skeptical caution would result in an intolerable
fragmentation of science, Hempel argued. It would result in "a
proliferation of concepts of length, of temperature, and of all other
scientific concepts that would not only be practically unmanageable,
but theoretically endless." Hempel's worry was that
Bridgman's quest for safety was blinding him to one of the
ultimate aims of science, "namely the attainment of a simple,
systematically unified account of empirical phenomena" (Hempel
1966, 94). Along similar lines, Lindsay (1937, 458) had earlier argued
that "such an isolation of concepts would defeat the very aim of
physical science, which is to provide a simple and economical
description" of physical experience "in terms of a minimum
number of concepts". Bridgman had serious doubts about the
plausibility of such a simple, unified account of nature, as I will
explain further in Section 3.4. But Hempel and others could genuinely
envisage it. Hempel noted that with the development of science there
was a continually growing and thickening network of "nomic
threads" linking various "knot-concepts" with each
other as further empirical laws were discovered. Hempel argued that it
was essential to keep this thickening conceptual network systematic
and simple; to that end, "concept formation and theory formation
must go hand in hand" (Hempel 1966, 97). That, in turn, often
necessitated "a modification of the operational criteria
originally adopted for some of the central concepts" (Hempel
1966, 95). Operationalism would stand in the way of such
flexibility.


### 2.3 What are operations?



Apart from the questions of whether operational definitions are
sufficient or necessary, it is actually unclear what types of things
operations *are*, and how they should be specified. This
problem was noted and discussed quite early on (see, for example,
Hearnshaw 1941). The surface-level intuition is simple: the operations
that matter are measurement operations involving physical instruments.
But from the start Bridgman (1927, 5) also stated that the operations
which fixed meaning were mental if the concepts in question were
mental (e.g., in mathematics). And he knew that measurement operations
involved more than physical manipulations of instruments; at least
there are recordings and computations involved in the processing and
analysis of data, and there are mental acts linking various parts of
that complex procedure, too. To take the simplest example, the
operation of counting is a mental operation, but it is an integral
part of many "physical" procedures. He called such crucial
non-physical operations "paper-and-pencil" operations.
Bridgman lamented that it was the "most wide spread
misconception with regard to the operational technique" to think
that it demanded that all concepts in physics must find their meaning
only in terms of physical operations in the laboratory (Bridgman 1938,
122-124; also Bridgman 1959b, 522). Later he gave a rough
classification of operations into the instrumental, mental/verbal, and
paper-and-pencil varieties (Bridgman 1959a, 3).



This issue becomes sharper when we ask the question of purpose: what
are the aims of operational analysis, and which operations are
suitable for achieving those aims? Having distinguished various types
of operations, Bridgman also had to deal with the question of whether
the different types had different epistemic values, going beyond his
initial intuitive fondness for instrumental operations. For example,
if the point of operationalizing a concept was to make its meaning
clear and precise, which meant using "operations which can be
unequivocally performed" (Bridgman 1938, 119), then why were
paper-and-pencil operations such as the construction of Euclidean
geometric figures not just as good as instrumental operations? In the
end he was willing to dispense with any ultimate privileging of
instrumental operations. But he still maintained a preference for them
when possible, without giving a convincing reason for that preference
(Bridgman 1938, 127).



Thus Bridgman's position on the nature and function of
operations was uneasy from start to finish. Various critics rightly
zeroed in on this point. The most important point of contention was
whether and why physical or instrumental operations had any special
epistemic advantage. The Yale physicist Henry Margenau put the point
succinctly:



> 
> Operationalism is an attitude that emphasizes the need of recourse,
> wherever feasible, to instrumental procedures when meanings are to be
> established. Bridgman disavows its status as a philosophy, and wisely
> so, for as a general view .... it cannot define the meaning of
> "instrumental procedure" in a manner that saves the view
> from being either trivial (which would be true if
> "instrumental" were construed to include symbolic, mental
> and paper-and-pencil operations) or too restrictive (if all operations
> are to be laboratory procedures). (Margenau in Frank 1956, 45)
> 



As one can imagine, this dilemma also hampered attempts to use
operational analysis in psychology. Operations in psychological
research inevitably involved verbal instructions, reports and
reactions. It was difficult to argue that these mental or verbal
operations were superior in reliability or meaningfulness to the
introspective reporting of mental states, which operationists tried so
hard to exclude from scientific psychology.



Bridgman himself was troubled by the question regarding the nature of
operations and admitted late in his life that he had not really
provided "an analysis of what it is that makes an operation
suitable", or "in what terms can operations be
specified" (Bridgman in Frank 1956, 77). An even deeper
pessimism was expressed in Bridgman's 30-year retrospective on
*The Logic of Modern Physics*, commissioned for
*Daedalus* by Holton: "To me now it seems
incomprehensible that I should ever have thought it within my powers
... to analyze so thoroughly the functioning of our thinking
apparatus that I could confidently expect to exhaust the subject and
eliminate the possibility of a bright new idea against which I would
be defenseless" (Bridgman 1959b, 520).


### 2.4 Are operations private or public?



One last issue needs to be mentioned, before I complete the discussion
of the critique of operationalism: the privacy of operations. This is
perhaps not widely remembered now, but it was the issue on which
Bridgman's position elicited the most severe incomprehension and
objection, even from many who called themselves operationalists.



The emblematic moment in this dispute came during the 5th
International Congress for the Unity of Science in 1939, held at
Harvard University--one of the highpoints of the activities of
the "Vienna Circle in exile" in America (see Holton
1995b). Bridgman was invited to address this conference, and chose to
give a talk entitled "Science: Public or
 Private?".[4]
 At this point it became clear that his enterprise was fundamentally
at odds with the logical positivist project, despite the surface
kinship:



> 
> The process that I want to call scientific is a process that involves
> the continual apprehension of meaning, the constant appraisal of
> significance, accompanied by a running act of checking to be sure that
> I am doing what I want to do, and of judging correctness or
> incorrectness. This checking and judging and accepting that together
> constitute understanding are done by me, and can be done for me by no
> one else. They are as private as my toothache, and without them
> science is dead. (Bridgman 1955, 56)
> 



Positivists and behaviorists had embraced operationalism for precisely
the opposite reason: they thought operations were public, objective
and verifiable, unlike private experience. But Bridgman was insistent
that operations were a matter for private experience. He could see no
warrant in simply taking someone else's testimony as true or
reliable, or in regarding the report of an operation performed by
someone else as the same kind of thing as an operation performed and
experienced by himself. In a later paper called "New Vistas for
Intelligence" he declared: "Science is not truly objective
unless it recognizes its own subjective or individual aspects"
(Bridgman 1955, 556). As Holton puts it (2005, 74), Bridgman's
drive in operational analysis was "to throw the spotlight on
performable action, above all an action performed by himself.
Ultimately, he was a private man, so much so that he was accused of
solipsism, to which he scarcely objected." In his epistemic
individualism Bridgman was perhaps only matched by Herbert Dingle, the
founder of the British Society for the Philosophy of Science, among
notable 20th-century philosophers of science (see Dingle 1950).



Bridgman's individualistic bent, both in epistemology and social
life, was in stark contrast to the logical positivist vision of
knowledge and society, particularly the strand of positivism driven by
Otto Neurath (1882-1945). The latter's aversion to the
private compelled him to express even first-hand observation reports
as third-person happenings in space and time, of the following type:
"Otto's protocol at 3:17 o'clock: [Otto's
speech-thinking at 3:16 o'clock was: (at 3:15 o'clock
there was a table in the room perceived by Otto)]." (Neurath
1932-33 [1983, p. 93]). Bridgman was unyielding in his
opposition to Neurath's type of objectification of personal
experience. To him, operations provided the best possible refuge from
the ocean of uncertainty always threatening to engulf science, and the
relative certainty was only possible if he was learning from his own
operations, not from second-hand reports from some other person. In
this regard Bridgman was closer to the strand of logical positivism
represented by Moritz Schlick (1882-1936), who insisted on
maintaining the notion of direct experience as the final arbiter of
knowledge. Schlick (1930 [1979]) admitted that experience was fleeting
and only provided momentary points of verification rather than any
lasting "foundation" one could build knowledge on.
Bridgman's operations had more promise in this regard, as the
operations were meant to be repeatable so the descriptions of
operations and their results would be lasting. However, that was not
to be such a straightforward matter, as we will see in Section
3.4.



As Holton (1995a; 2005) reports from his first-hand observations, the
privacy of operations (and the consequent privacy of science) was not
an idle philosophical doctrine for Bridgman. In the lab he carried out
as much of the work as possible with his own hands, using few
assistants and crafting most of his instruments himself. Holton
(1995a, 222-223) quotes the following report as typical of the
way Bridgman worked: "It is easy, if all precautions are
observed, to drill a hole ... 17 inches long, in from 7 to 8
hours"--that is, a hole as narrow as the lead in a pencil,
in a block of very hard steel. In academic life Bridgman (1955, 44)
openly lamented the "intellectual fashion ... of
emphasizing that all our activities are fundamentally social in
nature". As for his social and political writings, they were
often agonizing attempts to clarify, for himself, the place of the
"intelligent individual" in society. He was unabashedly
elitist, both on behalf of the gifted individual and of scientists as
a group, and argued that giving appropriate special treatment to
scientists would in the end benefit society (that is, all the
individuals in society). Maila Walter observes (1990, 192-193):
"Within the community of scientists and scientific philosophers,
Bridgman had become the lone spokesman for a radical existential
subjectivism", more akin to Rheinhold Niebuhr's
existentialist theology than to any commonly recognized philosophy of
science. Bridgman's uncompromising individualism continued right
to the end, with a self-administered euthanasia in the late stage of a
painful terminal illness (see Holton 1995a, 226-227).


## 3. Current relevance of operationalism



Is operationalism only a historical curiosity? In this final section,
I would like to give a synoptic view of the relevance of
operationalism to some issues that are current in philosophy of
science.


### 3.1 Operations as units of analysis of scientific practice



As already noted, Bridgman's ideas first gained recognition in
the midst of the logical-positivist preoccupation with language and
meaning; therefore, operationalism was taken primarily as a doctrine
about meaning and, as such, shown to be inadequate. In that context,
it was reasonable for most philosophers to abandon it. Bridgman made
various attempts to get beyond the popular caricature of
operationalism adopted by advocates and critics alike. These attempts
never received sufficient attention, but they offer some valuable
lessons and show many productive directions in which his views can be
interpreted and extended.



It is useful to continue listening to his retrospective given in the
1953 AAAS conference, quoted earlier. Bridgman tells us that what he
advocated was merely:



> 
> an attitude or point of view generated by continued practice of
> operational analysis. So far as any dogma is involved here at all, it
> is merely the conviction that it is better, because it takes us
> further, to analyze into doings or happenings rather than into objects
> or entities. (Bridgman in Frank 1956, 76)
> 



Already in his paper on "Operational Analysis" Bridgman
(1938, 115-116) had stated that in the attempt to understand how
science works, "the subject matter ... is activity of one
sort or another". He equated "activity" and
"operations", the term "operation" only
accentuating the directedness of the activity in question. In his last
general philosophical treatise, *The Way Things Are*, Bridgman
came back to this theme and stated that an operational analysis was
only "a particular case of an analysis in terms of
activities--doings or happenings", instead of analysis
"in terms of objects or static abstractions", or "in
terms of things or static elements" (1959a, 3; also 1959b,
522).



If we take our inspiration from this later Bridgman, we can take him
as a guide for a new practice-oriented philosophy of science. We can
put aside his reductive doctrine of meaning, his puritanical search
for certainty, and his ambivalent privileging of instrumental
operations over other types of operations. What Bridgman started up
but never quite accomplished in a systematic and complete way was a
philosophical analysis of science in terms of activities. Operations
provide the philosopher (and the historian) of science with a very
useful unit of analysis: actions or events, as opposed to objects,
statements, beliefs, theories, paradigms, research programs, etc. The
concept of operation should provide an effective framework for
incorporating certain highly valuable insights about the nature of
scientific practice, including the ideas of Ludwig Wittgenstein (1953)
on language-games, Michael Polanyi (1958) on tacit knowledge, Marjorie
Grene (1974) on the knowing agent, and Ian Hacking (1983) on direct
interventions in experimental investigations.



In order to develop Bridgman's operational analysis into a
full-fledged philosophy of scientific practice, there are some aspects
of his thought that we need to develop and articulate further. First
of all, as noted in Section 2.3 above, we need a clearer and more
detailed taxonomy of operations, without trying to say which types are
better or worse at the outset. From this point of view it should not
be regarded as a problem or an annoyance that there are different
types of operations. The categories offered by Bridgman are much too
broad, so we need to identify and describe specific and concrete
operations, and also distinguish simple and elementary ones from more
complex ones. For example, the operation of length measurement with a
meter-stick would be analyzable into the instrumental operations of
alignment and concatenation, the perceptual operation of judging
spatial coincidence, and the mental operation of counting. The
operation of hypothesis-testing (in the "received view")
would be analyzable into the simpler operations of deductive
prediction, experimental observation, and the comparison between
deduced and observed outcomes. In the understanding of these
operations we would need a detailed account of the scientist as the
agent who performs the operations; here we return to Bridgman's
preoccupation with the free individual, but also in essential social
interaction with other individuals. A full understanding of operations
would require an understanding of the agent's purposes (partly
based on the fundamental aims of science), assumptions (including
metaphysical principles essential for the particular type of activity
in question), and skills and capabilities (including the tacit
dimension). If we can achieve such a thick description of the
operations that constitute scientific practice, we would be able to
make good on Bridgman's promise that "it is better,
because it takes us further, to analyze into doings or happenings
rather than into objects or entities."


### 3.2 Operationalism as a philosophy of extension



Above I presented a new operationalism as a promising framework for
the analysis of scientific practice. Do Bridgman's ideas and
attitudes also hold any current relevance for practicing 
 scientists?[5]
It should have been clear from the discussion in Section 1 above that
Bridgman intended to reform scientific practice itself, not just its
second-order analysis, so we must ask whether his reformist agenda has
anything left in it for current science. To the casual reader, much of
Bridgman's writing will seem like a series of radical complaints
about the meaninglessness of various concepts and statements. But he
was not interested in skeptical critique as an idle and indiscriminate
philosophical exercise. He got most worried when a concept was being
extended to new situations where the familiar operations defining the
concept ceased to be applicable. His arguments often had an
iconoclastic flavor because he was exceptionally good at recognizing
where a concept had been extended to new domains unthinkingly and most
people were not even aware that the extension had been made. From the
methodological lesson he took from Einstein to the insights gained in
his own high-pressure physics, an important focus of Bridgman's
operationalism was on regulating the extension of concepts to
uncharted domains.



Bridgman reminded us forcefully that measurement operations did not
have unlimited domains of application, and that our conceptual
structures consequently had "joints" at which operational
meanings changed. But there can be no "joints" if there is
no continuous tissue around the disjointed bones. Less metaphorically:
if we reduce meaning entirely to measurement operations, there are no
possible grounds for assuming or demanding any continuity of meaning
where there is clear discontinuity in measurement operations. When we
have two different operations which give convergent results in the
overlapping domain, how do we tell whether what we have is an
accidental convergence of the measured values of two unrelated
quantities, or a unified concept measured by two different methods?
Some critics have maintained that only a recourse to theories can give
us the answer (e.g., Lindsay 1937, 458; Gillies 1972, 23). That does
not seem to me always necessary, as theory is not the only source of
semantic continuity. There are instrumental operations that are not
metrological, and these operations can provide a continuity of meaning
against which metrological validity can be judged (see Section 2.1).
We can take operationalism as a useful and practicable caution not to
make conceptual extensions without operational grounds.



A homespun example from the 18th century illustrates this point
nicely: the efforts of the English potter Josiah Wedgwood
(1730-1795) to extend the temperature scale to cover the very
high temperatures in his kilns, at which mercury vaporized and glass
melted. All previously known thermometers failed in that pyrometric
range, so Wedgwood felt obliged to invent a whole new measurement
standard (reminiscent of Bridgman in his high-pressure laboratory).
Wedgwood noticed that very high temperatures made pieces of clay
shrink, and created a temperature scale by assuming that the amount of
contraction was proportional to temperature beyond "red
heat". As the start of his scale (red heat, defined as 0) was
already beyond the boiling point of mercury, Wedgwood's scale
was wholly disconnected from the temperature scale defined by mercury
thermometers. Later, in response to widespread demand to clarify the
meaning of his scale in more usual terms, Wedgwood made a translation
of his scale into Fahrenheit degrees, by means of an intermediate
standard (thermal expansion of silver) which overlapped with the high
end of the mercury scale and the low end of the clay scale. (This
procedure produced some unlikely numbers, for example 21,877degF for
the temperature of his air-furnace.) It seems that Wedgwood initially
did exactly what operationalist conscience would dictate: as the new
instrument did not operate at all in the range of any trustworthy
previous thermometers, he made a fresh scale. Why was that not the
honest thing to do, and quite sufficient, too? Why did everyone,
including Wedgwood himself, feel compelled to interpret the Wedgwood
clay scale in terms of the Fahrenheit mercury scale? Why was a
continuous extension desired so strongly, when a disjointed set of
operations seemed to serve all necessary practical purposes?



The urge for conceptual extension, in the Wedgwood case, was rooted in
a widespread feeling that there *was* a property in the
pyrometric range that was continuous in its meaning with temperature
in the everyday range. Where did that feeling come from, long before
there was any well-defined and agreed-upon theoretical concept of
temperature? If we look closely at the situation, numerous subtle and
often-unspoken connections do emerge between pyrometric temperature
and everyday temperature. In the first place, we do bring objects to
pyrometric domains by prolonged heating--that is to say, by a
prolonged application of ordinary processes that cause the rise of
temperature within the everyday domain. Likewise, the same causes of
cooling that operate in the everyday domain will bring objects from
pyrometric temperatures down to everyday temperatures; that is
precisely what happens in calorimetric pyrometry (or, when we simply
leave very hot things out in cold air for a while). These concrete
physical operations provide a continuity of operational meaning
between two domains that are not connected by a common measurement
standard. Here again we need to articulate something that Bridgman
already did imply: not all instrumental operations are measurement
operations (for instance, we may know how to make iron melt without
thereby obtaining any precise idea of the temperature at which that
happens). Operational meaning even in the narrow, instrumental sense
is broader than meaning specified by methods of measurement.



The connections listed above rest on very basic qualitative causal
assumptions about temperature: fire raises the temperature of any
ordinary objects on which it acts directly; if two objects at
different temperatures are put in contact with each other, their
temperatures tend to approach each other. There are semi-quantitative
links as well. It is taken for granted that the consumption of more
fuel should result in the generation of more heat, and that is partly
based on a primitive notion of energy conservation. It is assumed that
the amount of heat communicated to an object is roughly proportional
to the amount of change in its temperature (barring changes of state
and interfering influences), and that assumption is based on the rough
but robust understanding of temperature as the "degree of
heat." So, for example, when a crucible is put on a steady fire,
one assumes that the temperature of its contents will rise steadily,
up to a certain maximum. That is exactly the kind of reasoning used by
the chemist John Frederic Daniell (1790-1845) to criticize some
of Wedgwood's results:



> 
> Now, any body almost knows, how very soon silver melts after it has
> attained a bright red heat, and every practical chemist has observed
> it to his cost, when working with silver crucibles. Neither the
> consumption of fuel, nor the increase of the air-draught, necessary to
> produce this effect, can warrant us in supposing that the fusing point
> of silver is 4 1/2 times higher than a red heat, fully visible in
> day-light. Neither on the same grounds, is it possible to admit that a
> full red-heat being 1077deg[F], and the welding heat of iron
> 12,777deg[F], that the fusing point of cast iron can be more than
> 5000deg higher. The welding of iron must surely be considered as
> incipient fusion. (quoted in Chang 2004, 149)
> 



Similar types of rough assumptions were also used in the extension of
temperature to very low temperatures (beyond the freezing point of
mercury and alcohol).



These cases illustrate that concepts can and do get extended to fresh
new domains in which theories are uncertain and experience scant, even
if no definite measurement operations have been worked out. We start
with a concept with a secure net of uses giving it stable meaning in a
restricted domain of circumstances. The extension of such a concept
consists in giving it a secure net of uses, also credibly linked to
the earlier net, in an adjacent domain. Such extension can happen in
all kinds of ways, including theoretical fiat and metaphysical
presumption, but the operational method is the most assured one.
Specific, well-defined operations, whether they be instrumental,
mental or paper-and-pencil type, can start a secure skeleton of
meaning in the new domain. With all the elements of new meaning
operationally well-defined, it also becomes possible to attempt
linking them up with each other at each step along the way and check
the whole meaning for coherence. (Compare such a deliberate process
with the vague presumption that terms in a theoretical equation must
have the same meaning in the entire mathematical range given to the
variables.) Operationalism in this guise can be used as a secure
method of conceptual extension, quite the opposite of the kind of
fragmentation that Hempel feared. Such operationalism would not
destroy systematic unity; on the contrary, it is an optimal strategy
for achieving as much systematic unity as nature would allow, in a
strongly empiricist system of knowledge.


### 3.3 Operationalism as a strategy for increasing empirical content



Conceptual extension is important, especially since it served as one
of the key initial motivations of Bridgman's thought, but it is
only part of the operationalist story. In more general terms,
operationalism can be seen as a strategy for increasing the empirical
content of scientific theories. What is empirical content? Karl Popper
saw the amount of the empirical content of a theory as the number of
states of the world that are forbidden by it. Regarding laws of nature
he said: "the more they prohibit they more they say"
(Popper 1972, 41). Or, somewhat more formally: "I define the
*empirical content* of a statement *p* as the class of
its potential falsifiers" (120). Similarly but staying away from
the strict falsificationist idiom, Imre Lakatos understood empirical
content as the number of empirically testable predictions. Empirical
content is not something we hear about very much these days in
philosophy of science after the receding of Popperian and Lakatosian
doctrines, but for Bridgman and many other operationalists it was one
of the key issues.



If we take operationalism as a commitment to increase empirical
content, Bridgman was not so much a high-handed judge who pronounced
upon the meaningfulness of concepts in a black-and-white manner.
Rather, he offered operational analysis as a tool of self-diagnosis
and self-improvement. He was interested in advancing science, not in
carping against it; like Descartes, he used skepticism as a means of
achieving a more positive end. In this connection there is a point to
Bunge's (1988, 341) contention that "operational
definition" is a misnomer and we should instead speak of
"indicator hypotheses" that offer links between
unobservable properties and observable ones. In my view,
Bridgman's enterprise can be understood quite well as an
instance of what goes by the name of "conceptual
engineering" these days (see Cappelen 2018, and Brun 2016 for
connections with Carnapian explication). Concepts can be engineered to
various ends, and in the case of operationalism the overriding concern
is to make them as empirically meaningful as the circumstances
permit.



The operationalist dictum could be phrased as follows: maintain and
increase the empirical content of theories by the use of operationally
well-defined concepts. It is difficult to craft an exact quantitative
measure of empirical content, but we can at least say that the amount
of empirical content depends on the number of empirically testable
relations that a theory specifies. That, in turn, depends on the
number of independently measurable parameters. Increasing the number
of independent parameters, or at least maintaining it, was something
that Bridgman sought to achieve with his operationalism. This, I
submit, was one of the key reasons why he did not like conceptual
extensions that were not backed up by measurement operations in the
new domain. And it seems to me that this lesson was taken to heart by
the operationist psychologists much more than the physicists, as Feest
(2010; 2012) explains in her discussion of how operational definitions
should be understood as tools of experimental investigation.



To follow Bridgman's thinking along these lines, consider this
intriguing passage, which at first glance looks like another complaint
about meaninglessness. But towards the end the main point emerges as a
worry about diminishing empirical content:



> 
> What is the possible meaning of the statement that the diameter of an
> electron is 10-13 cm? Again the only answer is found by
> examining the operations by which the number 10-13 was
> obtained. This number came by solving certain equations derived from
> the field equations of electrodynamics, into which certain numerical
> data obtained by experiment had been substituted. The concept of
> length has therefore now been so modified as to include that theory of
> electricity embodied in the field equations, and, most important,
> assumes the correctness of extending these equations from the
> dimensions in which they may be verified experimentally into a region
> in which their correctness is one of the most important and
> problematical of present-day questions in physics. To find whether the
> field equations are correct on a small scale, we must verify the
> relations demanded by the equations between the electric and magnetic
> forces and the space coordinates, to determine which involves
> measurement of lengths. But if these space coordinates cannot be
> given an independent meaning apart from the equations, not only is the
> attempted verification of the equations impossible, but the question
> itself is meaningless. If we stick to the concept of length by itself,
> we are landed in a vicious circle. As a matter of fact, the concept of
> length disappears as an independent thing, and fuses in a complicated
> way with other concepts, all of which are themselves altered thereby,
> with the result that the total number of concepts used in describing
> nature at this level is reduced in
>  number.[6]
>  (Bridgman 1927, 21-22)
> 



Such a reduction in the number of operationally meaningful concepts
will almost inevitably result in a corresponding reduction in the
number of relations that can be tested empirically. A good scientist
would fight against such a prospect of diminished empirical
content.



This concern with empirical content also explains why Bridgman was not
content with the mainstream post-positivist philosophical discourse on
concept-formation and empirical significance, exemplified by the works
of Carl Hempel and Willard Van Orman Quine. As noted in Section 2.2,
Bridgman did not object to theoretical science creating a system of
concepts and laws that made contact with observations only at some
points. However, Quinean holism, in which the unit of empirical
significance was the entire system of knowledge, had no particular
concern about increasing the number of those points-of-contact with
experience. Bridgman's ideal was to operationalize each and
every concept if possible, and each case of de-operationalization rang
alarm bells in his head.



Recognizing the importance of empirical content helps us make sense of
Bridgman's complex attitude toward theoretical concepts. In a
little-known section of *The Logic of Modern Physics*, he
discussed what he called "mental constructs" in science,
particularly those created in order to "enable us to deal with
physical situations which we cannot directly experience through our
senses, but with which we have contact indirectly and by
inference" (1927, 53-60). Not all constructs are the
same:



> 
> The essential point is that our constructs fall into two classes:
> those to which no physical operations correspond other than those
> which enter the definition of the construct, and those which admit of
> other operations, or which could be defined in several alternative
> ways in terms of physically distinct operations. This difference in
> the character of constructs may be expected to correspond to essential
> physical differences, and these physical differences are much too
> likely to be overlooked in the thinking of physicists. (Bridgman 1927,
> 59-60)
> 



They were also very easily overlooked in the thinking of
*philosophers* who debated his ideas. What Bridgman says here
is entirely contrary to the common image of his doctrines. When it
came to constructs, "of which physics is full," Bridgman
not only admitted that one concept could correspond to many different
operations, but even suggested that such multiplicity of operational
meaning was "what we mean by the reality of things not given
directly by experience." In an illustration of these ideas,
Bridgman argued that the concept of stress within a solid body had
physical reality, but the concept of electric field did not, since the
latter only ever manifested itself through force and electric charge,
by which it was defined (Bridgman 1927, 57). This comes down to the
stance that a theoretical concept without direct operational meaning
is worthwhile only if it serves as a mediator connecting two or more
operationally meaningful concepts, creating an empirically testable
relation. This is in fact not so different from Hempel's view
quoted in Section 2.2, albeit with a different emphasis.


### 3.4 Operational analysis as the revealer of complexity



In closing, I would like to pull out an insight from Bridgman that is
usually not recognized in discussions of operationalism, but actually
has emerged as a significant point of contention in more recent
philosophy of science. This is the issue of *complexity*. In
Section 1.1 I have already quoted Bridgman's puzzling statement
that nature is ultimately "neither understandable nor subject to
law." As it turns out, that was not an isolated off-hand remark.
An important aspect of Bridgman's operationalism was a search
for certainty, and it was a search made all the more desperate by a
deep-rooted pessimism about the possibility of attaining any certainty
in science, at least if scientists were to seek a simple and unified
system of knowledge. Bridgman professed his belief that "the
external world of objects and happenings is ... so complex that
all aspects of it can never be reproduced by any verbal
structure." He lamented: "Even in physics this is not
sufficiently appreciated, as is shown, for example, by the reification
of energy. The totality of situations covered by various aspects of
the energy concept is too complex to be reproduced by any simple
verbal device." (Bridgman in Frank 1956, 78)



Bridgman's view on the complexity of nature also had a direct
implication for the limits of operational analysis itself in providing
clarity and precision. Right from *The Logic of Modern
Physics*, Bridgman stressed that "all results of measurement
are only approximate"; this obvious fact, he said,
"tacitly underlies all our discussion". This he attributed
ultimately to something fundamental about the nature of human
experience: "all experience seems to be of this character; we
never have perfectly clean-cut knowledge of anything, but all our
experience is surrounded by a twilight zone, a penumbra of
uncertainty, into which we have not yet penetrated. This penumbra is
as truly an unexplored region as any other region beyond
experiment" (1927, 33). This indicated a fundamental limitation
to the certainty of operations: "Operations themselves are, of
course, derived from experience, and would be expected also to have a
nebulous edge of uncertainty" (1927, 36). Bridgman remained
clearly aware of the complexities revealed by operational analysis,
stating late in his life that "operational analysis can always
be pushed to the point where sharpness disappears" (Bridgman in
Frank 1956, 78), and that "there is nothing absolute or final
about an operational analysis" (Bridgman 1959b, 522). Still, he
would not give up on the pushing, which was necessary to reach as much
clarity as possible.



Bridgman's battle against his own skeptical and pessimistic
conscience was a heroic one. After decades of operationalist thinking
he arrived at "a picture of man isolated ... in an oasis of
phenomena which he will never be able to transcend because beyond its
bounds the operations are impossible which are necessary to give
meaning to his thought" (Bridgman 1955, 540). What this picture
forced on him was a deep sense of humility, as expressed in his
memorable statement on the scientific method: "The scientific
method, as far as it is a method, is nothing more than doing
one's damnedest with one's mind, no holds barred"
(1955, 535). Retaining that sense of humility will help us in
developing Bridgman's unfinished thoughts to create a new
operationalism that does full justice to the complexity and richness
of both nature and human scientific practice.