In pursuit of formaldehyde: Causally explanatory models and falsification

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Abstract

Falsification no longer is the cornerstone of philosophy of science; but it still looms widely that scientists ought to drop an explanatory hypothesis in view of negative results. We shall argue that, to the contrary, negative empirical results are unable to disqualify causally explanatory hypotheses—not because of the shielding effect of auxiliary assumptions but because of the fact that the causal irrelevance of a factor cannot empirically be established. This perspective is elaborated at a case study taken from the history of plant physiology: the formaldehyde model of photosynthesis, which for about sixty years (1870s to 1930s) dominated the field—despite the fact that in these sixty years all the attempts to conclusively demonstrate even the presence of formaldehyde in plants failed.

Introduction

While for a long time the discussion of “theories” was dominant in the philosophy of science, in the last decades the literature on scientific models, as something in between theories and rules of the thumb, has proliferated enormously.1 In this paper, we focus on causal models. These models explain sequences of events by spelling out the relevant factors which produce the effects in question and define their relationships to each other. We take the appropriate visual representation of such models to be causal graphs consisting of a network of nodes (which indicate causal factors) and directed edges (which indicate causal relationships). Prime examples of this type of model are biochemical pathways, which perhaps not coincidentally are frequently depicted as graphs by the scientists themselves. Biochemical pathways describe the stepwise development of products out of a series of starting materials; they may take the form of a long chain of reactions or the form of a cycle; they are often very complex, while for many purposes simplified versions do well, since they can be expanded as occasion and knowledge demand (while the absence of a factor in the modelled pathway does not imply the factor’s irrelevance). How these pathways are established from experimental results, how they are modified and adapted is, therefore, of tremendous interest if one wants to learn more on the question how scientific models are construed—and under which circumstances they are abandoned.

The latter question is studied in this paper by the example of an episode taken from the long-winded search for the biochemical pathway of photosynthesis. This process in which solar energy is converted into energy that can be used biochemically is fundamental to life on earth, and the way organisms accomplish this task has intrigued scientists for more than two centuries. Yet still around 1900, photosynthesis was basically a black box, the internal mechanism of which was totally obscure. Scientists knew the starting materials and the products of the process, but they had only vague hypotheses of what happened in between.2 The model which most scientists favoured at the end of the nineteenth century was the formaldehyde model of photosynthesis. Originally suggested in 1870, it dominated the field until the 1930s, that is, for sixty years, although alternative models had always been debated.3 The puzzling fact is that the model remained dominant despite of the fact that none of the almost innumerable attempts to conclusively demonstrate the presence of the key intermediate—formaldehyde—in the green parts of plants, had ever been successful. From the point of view of traditional philosophy of science, these failures should have counted as an instance of fatal falsification, if ever there was one.

We shall argue that this way of thinking reveals a serious misconception of the nature of the underlying explanatory model. If models of biochemical pathways, such as the photosynthetic production of glucose, are understood as a framework of causal hypotheses (as opposed to simple conditionals), it becomes immediately clear why they cannot be falsified by negative empirical results. This impasse is closely related to Pierre Duhem’s well-known non-falsifiability thesis. Duhem drew attention to the fact that it is impossible to test (and reject) a specific hypothesis in isolation, since empirical predictions are always based on an entire system or group of hypotheses, and, on top of that, the derivation requires a number of auxiliary hypotheses concerning the experimental setup. Therefore, the well-targeted falsification of one specific hypothesis is rendered impossible.4 Despite this powerful criticism, philosophy of science for a long time favoured falsificationism as the best—in fact, the only—way to advance empirical science, in line with the influential suggestions by Karl Popper.5 This tradition, however, also thought of scientific hypotheses as universal conditionals, whereas we would like to suggest, in contrast, that they are causal hypotheses.

Falsifying a causal hypothesis requires to prove a factor’s irrelevance for an effect. However, from the point of view of causal reasoning, this is impossible to do. It would require a complete grasp of the causal structures underlying the effect in question and this clearly is something human beings cannot even hope to achieve In contrast to this difficulty, there are well-trodden ways to constructively establish causal hypotheses—which, incidentally, is also far more interesting and useful than the demonstration of what is wrong.

Section snippets

Accepted body of knowledge

We shall start by introducing some background to the example discussed in this paper. The well-known equation for oxygenic photosynthesis formulates the process as follows:6CO2+6H2OC6H12O6

This equation, which was defined in the nineteenth century, contains the commonly held core assumptions on the processes of photosynthesis that were considered beyond dispute: that carbon dioxide and water (or the combination of these compounds in form of carbonic acid) are the starting materials of the

The model

The formaldehyde model was first put forward in 1870 by the German organic chemist Adolf von Baeyer (1835–1917).8 One of the most eminent figures of his time, Baeyer is particularly renowned for his research on the plant dye indigo: Baeyer successfully synthesised this important dye in the test tube in 1880, and by 1883 he had completely elucidated the molecule’s structure. (Baeyer was awarded the Nobel Prize in

Causal reasoning from experimental results

We mentioned in the introduction that we take explanatory models, such as the formaldehyde model, to be aptly represented by causal graphs: complex (but partial) networks of nodes (causal factors) and directed edges (causal relationships between nodes). The only way to reliably establish causal hypotheses is by conducting difference tests, which are frequently carried out in the form of experiments. A difference test realises two situations which comply with the homogeneity condition: they are

The meaning of negative results

In addition to the attempts to prove formaldehyde’s existence in plants, that is, the instantiation of this factor, scientists also tried hard to prove its causal effect on photosynthesis. One can derive from the formaldehyde model the following hypothesis: “Since formaldehyde is a key intermediate in photosynthetic assimilation, the presence of formaldehyde should lead to the photosynthetic formation of glucose (while no glucose should be produced if formaldehyde is absent).” This hypothesis

Why the model was dropped eventually

Although the hunting for formaldehyde went on for decades, it did not go on forever. Eventually, the model was dropped; but this was not due to any process of falsification. By the end of the 1930s, two decisive developments had taken place. First, the thermodynamic side of the process had become the subject of study; and it became increasingly difficult to reconcile the formaldehyde model—which was relatively costly, energetically speaking—with the amount of energy available for the process.

Concluding remarks

How explanatory models of causal processes are construed, modified and eventually abandoned, such as the model of the biochemical pathway of photosynthesis, is one of the central questions of the philosophy of science. Much can be learned about these issues from the reconstruction of actual case studies from the history of science; and one example was given in this paper. With hindsight, the formaldehyde model of photosynthesis was completely flawed. It was far too simplistic; furthermore,

Acknowledgements

We would like to thank Raphael Scholl (Bern) who provided helpful comments on an earlier draft and to Margareta Simons (Lucerne) who edited the manuscript.

References (100)

  • F. Angelico et al.

    Sulla presenza della formaldeide nei succhi delle piante verdi

    Gazzetta Chimica Italiana

    (1913)
  • A. Bach

    Contribution à l’étude des phénomènes chimiques de l’assimilation de l’acide carbonique par les plantes à chlorophylle

    Comptes Rendus de l’Académie des Sciences

    (1893)
  • A.J.F.v. Baeyer

    Über die Wasserentziehung und ihre Bedeutung für das Pflanzenleben und die Gährung

    Berichte der Deutschen Chemischen Gesellschaft zu Berlin

    (1870)
  • D. Bailer-Jones

    Scientific models in philosophy of science

    (2009)
  • S.M. Baker

    Quantitative experiments on the effects of formaldehyde on living plants

    Annals of Botany

    (1913)
  • E.C.C. Baly et al.

    Photocatalysis. Part I. The synthesis of formaldehyde and carbohydrates from carbon dioxide and water

    Journal of the Chemical Society, Transactions

    (1921)
  • M. Baumgartner et al.

    Kausalität und kausales Schliessen. Eine Einführung mit interaktiven Übungen

    (2004)
  • D. Berthelot et al.

    Synthèse photochimiques des hydrates de carbone aux dépens des elements de l’anhydride carbonique dt de la vapeur d’eau, en l’absence de chlorophyll: synthèse photochimique des composés quaternaires

    Comptes Rendus de l’Académie des Sciences

    (1910)
  • M. Black

    Models and metaphors: Studies in language and philosophy

    (1962)
  • R. Boitreux

    Sur la nutrition du Trichoderma viride (Pers.) à partir du formol libre

    Comptes Rendus des Séances de la Société de Biologie

    (1920)
  • H.I. Browman

    Negative results (Theme Section)

    Marine Ecology Progress Series

    (1999)
  • A.M. Butlerov

    Bildung einer zuckerartigen Substanz durch Synthese

    Annalen der Chemie und Pharmazie

    (1861)
  • N. Cartwright

    How the laws of physics lie

    (1983)
  • R. Chodat et al.

    Nouvelles recherches sure les ferments oxydants. IX. De l’emploi de la peroxydase comme réactif de la photolyse par la chlorophylle

    Archives des Sciences Physiques et Naturelles

    (1915)
  • P. Duhem

    The aim and structure of physical theory

    (1991)
  • R. Emerson et al.

    The photochemical reaction in photosynthesis

    Journal of General Physiology

    (1932)
  • H. Euler et al.

    Zur Kenntniss der Zuckerbildung aus Formaldehyd

    Berichte der deutschen chemischen Gesellschaft

    (1906)
  • H. Euler et al.

    Über die Bildung von i-Arabinoketose aus Formaldehyd

    Berichte der deutschen chemischen Gesellschaft

    (1906)
  • A.J. Ewart

    On the function of chlorophyll

    Proceedings of the Royal Society of London. Series B

    (1915)
  • H.J.H. Fenton

    The reduction of carbon dioxide to formaldehyde in aequous solution

    Journal of the Chemical Society, Transactions

    (1907)
  • E. Fischer

    Über die Verbindungen des Phenylhydrazins mit den Zuckerarten. III.

    Berichte der deutschen chemischen Gesellschaft

    (1888)
  • E. Fischer

    Synthese der Mannose und Lävulose

    Berichte der deutschen chemischen Gesellschaft

    (1890)
  • E. Fischer

    Synthese des Traubenzuckers

    Berichte der deutschen chemischen Gesellschaft

    (1890)
  • E. Fischer

    Synthesen in der Zuckergruppe

    Berichte der deutschen chemischen Gesellschaft

    (1890)
  • E. Fischer

    Synthesen in der Zuckergruppe I (Vortrag, gehalten in der Sitzung der Deutschen chemischen Gesellschaft am 23. Juni 1890)

  • E. Fischer et al.

    Bildung von Acrose aus Formaldehyd

    Berichte der deutschen chemischen Gesellschaft

    (1889)
  • M. Florkin

    A history of biochemistry. Part IV. Early Studies on Biosynthesis. (Comprehensive Biochemistry, Vol. 32)

    (1977)
  • J. Franck

    Beitrag zum Problem der Kohlensäure-Assimilation

    Naturwissenschaften

    (1935)
  • Frigg, R., & Hartmann, S. (2006). Models in science. In E.N. Zalta (Ed.), The Stanford encyclopedia of philosophy...
  • H. Gaffron et al.

    Zur Theorie der Assimilation

    Naturwissenschaften

    (1936)
  • H. Gest et al.

    Time line of discoveries: anoxygenic bacterial photosynthesis

    Photosynthesis Research

    (2004)
  • R.J.H. Gibson

    A photoelectric theory of photosynthesis

    Annals of Botany

    (1908)
  • S.J. Gould

    Cordelia’s dilemma

    Natural History

    (1993)
  • Govindjee et al.

    Discoveries in oxygenic photosynthesis (1727–2003): A perspective

    Photosynthesis Research

    (2004)
  • V. Grafe

    Über ein neues specifisches Formaldehydreagenz

    Oesterreichische botanische Zeitschrift

    (1906)
  • V. Grafe et al.

    Untersuchungen über das Verhalten grüner Pflanzen zu gasförmigem Formaldehyd

    Berichte der deutschen chemischen Gesellschaft

    (1909)
  • V. Grafe et al.

    Untersuchungen über das Verhalten grüner Pflanzen zu gasförmigem Formaldehyd II

    Berichte der deutschen chemischen Gesellschaft

    (1911)
  • G. Graßhoff et al.

    Causal regularities

  • G. Graßhoff et al.

    Hans Krebs and Kurt Henseleit’s laboratory notebooks and their discovery of the urea cycle: Reconstructed with computer models

  • G. Graßhoff et al.

    Zur Theorie des Experiments. Untersuchungen am Beispiel der Entdeckung des Harnstoffzyklus

    (2000)
  • C. Hempel

    Philosophy of natural science

    (1966)
  • M.B. Hesse

    Models and analogies in science

    (1963)
  • H. Huzisige et al.

    Dynamics of the history of photosynthesis research

    Photosynthesis Research

    (1993)
  • E. Höxtermann

    Fundamental discoveries in the history of photosynthesis research

    Photosynthetica

    (1992)
  • M. Jacoby

    Über den Formaldehyd als Übergangsstufe zwischen der eigentlichen Assimilation und der Kohlenhydratbildung in der Pflanze

    Biochemische Zeitschrift

    (1919)
  • M. Jacoby

    Über den Formaldehyd als Übergangsstufe zwischen der eigentlichen Assimilation und der Kohlenhydratbildung in der Pflanze II

    Biochemische Zeitschrift

    (1922)
  • H. Kautsky et al.

    Energie-Umwandlungen an Grenzflächen. VI. Mitteilung: Kohlensäure-Assimilation (1.)

    Berichte der Deutschen Chemischen Gesellschaft

    (1932)
  • G. Kimpflin

    Présence du menthal dans les végétaux vertes

    Comptes Rendus de l’Académie des Sciences

    (1907)
  • G. Kimpflin

    Essai sur l’assimilation photochlorophyllienne du carbone

    (1908)
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