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Byron Jennings | TRIUMF | Canada

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The Siren Call of Logical Positivism

For every problem, there is a simple solution: neat, plausible and wrong.

The philosophers such as Rudolf Carnap (1891 – 1970) and the Vienna Circle considered logical positivism the received view of the scientific method.  In the early to mid twentieth century, it dominated the philosophy of science discussions but is now widely viewed as seriously flawed—or as A. J. Ayer (1910 – 1989), a former advocate, put it: “I suppose the most important [defect]…was that nearly all of it was false.” Pity. But it was good while it lasted. So, what is logical positivism? It is sometimes defined by the statement: Only verifiable statements have meaning—note verifiable not falsifiable. The doctrine included opposition to all metaphysics, especially ontology and synthetic a priori propositions. Metaphysics is rejected not as wrong but as having no meaning.

Logical positivism is very nice idea: we work only with observations and what can be deduced directly from them. No need for theories, models or metaphysics. I can hear the cheering now, especially from my experimental colleagues. It was partially in response to the revolutions in physics in the early twentieth century. Quantum mechanics and relativity completely upended the metaphysics and philosophy built around classical mechanics, so the logical positivist wanted to eliminate the metaphysics to prevent this from happening again; a very laudable goal.

So what went wrong? As Ayer noted, almost everything. First, metaphysics tends to be like accents—something only the other person has. The very claim that metaphysics is not needed is itself a metaphysical claim.  Second, observations are not simple. As demonstrated by optical illusions, what we see is not necessarily what is there.  The perceptual apparatus does a lot of processing before the results are presented to the conscious mind. The model of the universe presented to the conscious mind probably has more uncontrolled assumptions than any accepted scientific model. But that is what the logical positivists took as the gospel truth. In addition there is Thomas Kuhn’s (1922 – 1996) claim that observations are model dependent. While that claim is disputable, it is clear that the interpretation of observations depend on the model, the paradigm or if you prefer the metaphysics; something beyond the observations themselves.

Third as Sir Karl Popper (1902 – 1994) argued, in general, scientific models cannot be verified only falsified (and one can argue that even that is impossible, see the first post in this series).  Thus, Only verifiable statements have meaning would exclude all of science from having meaning. Indeed, it would exclude even the statement itself since the statement Only verifiable statements have meaning cannot be verified.

Logical positivism: neat, plausible and wrong. Well can anything be salvaged? Perhaps a little. Consider the statement: In science, only models that can be empirically tested are worth discussing. Not to be overly broad, I restrict the statement to science. The criteria in mathematics are rather different and I do not wish to make a general statement about knowledge, at least not here. Second, I have replaced statement with model since by the Duhem-Quine thesis individual statements cannot be tested since one can make almost any statement true by varying the supporting assumptions. In the end it is global models that are tested. Science is observationally based, so the adjective empirical. I use tested to avoid complaints about the validity of verification or falsification. Tested is neutral in that regard. Finally, meaningful has been replaced by worth discussing. To see why consider the composition of the sun. In the late nineteenth century, it was regarded as something that would never be known. At that point the statement “The sun is composed mainly of hydrogen” would have been considered meaningless by the logical positivists and certainly, at that time, discussion of the issue would have been futile. But with the discovery of spectroscopic lines, models for the composition of the sun became very testable and the composition of sun is now considered well understood. It went from not worth discussing to well understood but the composition of the sun did not change. I would consider the statement “The sun is composed mainly of hydrogen” to be meaningful even before it could be tested; meaningful but not worth discussing.

My restatement above does, however, eliminate a lot of nonsense; like the omphalos hypothesis, the flying spaghetti monster, and a lot of metaphysics, from discussion. But its implications are more wide ranging. During my chequered career as a scientist, I have seen many pointless discussions of things that could not be tested: d-state of the deuteron, off-shell properties, nuclear spectroscopic factors and various other technical quantities that appear in the equations used by physicists. There was much heat but little light. It is important to keep track of what aspects of the models we produce are constrained by observation and which are not. Follow the logical positivists, not the yellow brick road, and keep careful track of what can actually be determined by measurements. What is behind the curtain is only interesting if the curtain can be pulled aside.

To conclude: Don’t waste your time discussing what can’t be empirically tested. That is all that’s left of logical positivism once the chaff has been blown away. And good advice it is—except for mathematicians. Either that or I have been lured to the rocks by the siren call of logical positivism and have another statement that is neat, plausible and wrong!

Additional posts in this series will appear most Friday afternoons at 3:30 pm Vancouver time. To receive a reminder follow me on Twitter: @musquod



  • http://pseudomonad.blogspot.com Kea

    You know, I really do enjoy your little essays, but I always find myself frustrated by them. Please don’t confuse logical positivism with empiricism!

    No one would argue that the early 20th century style of positivism is outdated, but to my mind the central problem in Theory today is a false anti-positivism, which completely fails to understand the core issue. The case in point is Mach. Mach was of course an anti-atomist, and is much laughed at today for persisting with this view. Now most physicists today have little or no education in philosophy or history of science, and so fail to realise where Mach’s anti-atomism comes from. It is an important lesson. The problem was the entrenched materialist ontology of the time, where one could not speak of atoms without invoking discrete objects in an a priori background, aether if you will. Mach always understood that the basic components of reality could not come down to such an ugly ontology, and he was right, as any decent modern theorist would tell you. Measurement principles are more fundamenatal than an emergent spacetime, and although today this is no longer logical positivism as such, I think logical positivism is a helpful anchor from the past, in the destructive sea of a materialism that is long past its use-by date.

  • http://logicofnature.blogspot.com/ gunn

    It seems that LHC-experiments have falcified the basic models of theoretical physics (Higgs, a superstring and etc.). Now remains nothing except the logic analysis of the Standard Model.

  • http://www.triumf.ca/theory/byron-jennings Byron

    Glad you enjoy them. The problem with logical positivism is that they try to go directly from observation to meaning with the necessary intermediate step of a model.

  • http://pseudomonad.blogspot.com Kea

    You mean, WITHOUT the intermediate step? But if they truly did that, their theories and philosophies would be pure empiricism without any semantic depth, which they are not, at least not in the case of Mach! It is a question of what we permit to take on Meaning. You might argue that all true theories contain unobservable elements, and I would agree with that. But one can demand that the principles of the theory rely heavily on empirical fact, and that unobservable consequences are only derived features of the formalism. That seems very reasonable to me.

  • Torbjörn Larsson, OM

    Besides being a philosophical description, it sounds like an attempt to axiomatization. But that can’t be done in physics. (Say, quantization resists it, I am told.)

    “The very claim that metaphysics is not needed is itself a metaphysical claim.”

    It is is an observation. You know, what philosophy can’t handle, since you can’t falsify any philosophy. Thus philosophy has no bearing whatsoever on empirical matters.

    @ gunn:

    “Higgs … Standard Model”. This is inconsistent, since Higgs _is_ a part of the Standard Model.

    As it happens, LHC has not falsified anything yet. But have rather promising hints of a ~ 125 GeV Higgs.

  • http://logicofnature.blogspot.com/ gunn

    Torbjörn Larsson:

    I mean Cтандартную model Sheldon Glashow’s — SM without Higgs (http://www.pteponline.com/index_files/books_files/quznetsov2011.pdf).

    Similar hints since LEP was observed not less than three times – and what?

  • http://logicofnature.blogspot.com/ gunn
  • http://individual.utoronto.ca/fofound Allan Olley

    It should be said that Carnap and other logical positivists recognized the basic element of the Duhem-Quine thesis that one does not test individual statements but whole sets of principles (funnily enough Carnap attributed the basic observation that we test theories against sets of assumptions to Duhem and Poincaré). The logical positivists and the Vienna circle were unified more in their attempt to apply the successful methods of what they saw as best practice in science (and mathematics), their particular positions were at least somewhat diverse.

    For example Carnap (who I’ve been reading a little lately), actually takes something like the approach you suggest. He takes it that even empirical questions are only meaningful and decidable in a “language” (which is a complete system of description and inference and may include positing things like physical laws). He takes it as standard scientific practice to recognize these languages/assumption and work within them or switch to other ones where it is useful. So all non-empirical stuff just becomes a convention of the language assumed and therefore tautology, observations are not tautologies. So he does end up with something like the only empirical statements have meaning, but the inferences from observations are neither fish nor fowl (neither tautology nor simply empirically given). He would reject metaphysical statements for having no meaning, but more because say exist only has a meaning in a system and is only provable in a system (the species [genus?] Kangaroo “exist” in biology given biology’s classification scheme, standards of evidence and observations made is a perfectly sensible statement for Carnap), metaphysical statements become nonsense by being misapplied (the species Kangaroo “exist” by some non-biological sense of exists). Carnap advocated for a neutrality about languages and deciding which ones to use on pragmatic grounds. I think this is messy and if it does not end up as one of the post-positivist positions (Kuhn, Poppers or Quine’s) certainly threatens to.