The role of orientation experiments in discovering mechanisms

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Highlights

  • Orientation experiments are used to give a qualitative characterization of a mechanism.

  • They constrain future research and experimentation.

  • They are particularly useful during the early stages of investigation.

Abstract

Many types of experiments have been recognized in the literature. One important type we discuss in this article is the orientation experiment. While orientation experiments are like other types of experiments in that they are tests for causal relevance, they also have other qualities. One important (but not the only) goal of these experiments is to offer a rough, qualitative characterization of the mechanism responsible for a capacity of interest, effectively constraining future research. This makes them particularly useful during the early stages of investigation, when an explanandum-phenomenon has just been identified and several (often competing) hypotheses as to the qualitative character of the mechanism responsible for it are proposed. We illustrate our claims, and explicate a number of additional aims that orientation experiments can sometimes serve, by considering three case studies from different era's, namely the discovery of the mechanisms responsible for i) the capacity of eels to produce numbing sensations (17th and 18th century), ii) puerperal fever in Semmelweis' Vienna Maternity Hospital (19th century), and iii) the capacity of pigeons to home (20th century).

Introduction

This paper is about the way scientists discover mechanisms, in particular, about the reasoning employed that leads to such discoveries. Underlying discussions of these issues in the literature, is a general tendency to move away from the Popperian focus on the logic of discovery and justification, and towards a view of discovery as a problem-solving activity. According to Darden “Philosophers should move beyond talk of the (lack of) a logic of discovery and a logic of justification to study reasoning strategies for generation, evaluation, and revision in the discovery of mechanisms” (2009, p. 54).

Two dimensions are of importance to this project. First, there is the epistemological dimension of the process of discovering mechanisms. Here, Darden and others have developed a quite elaborate framework over the years, according to which this process can be partitioned into phases such as identifying the explanandum-phenomenon, the generation, the evaluation, and the revision of mechanistic hypotheses (Darden, 2009). Second, there are the types of reasoning strategies and experiments that are carried out during these stages of the investigation. Again, many different types of experiments and reasoning-patterns have been described—inter-level intervention experiments, as described by Craver (2007), constitute an important example. It would seem that in order to make progress, philosophers should concentrate on increasing this stock of strategies for discovery, and show how these strategies fit into the epistemological order (e.g. whether a strategy belongs primarily to the phase of revision, or is particularly useful when attempting to identify the explanandum-phenomenon). Regarding to the question as to where we should look, Darden suggests the history of science as a particularly promising source (2002, p. S364).

In this article, we will follow this suggestion. Inter-level experiments as envisaged come with certain presuppositions regarding the knowledge of the mechanism we must have before we can execute them. However, these presuppositions are not always met. Sometimes, the mechanism is largely unknown, or different mechanistic hypotheses might be competing. Instead of inter-level experiments, in these circumstances scientists perform a different kind of experiment. We call these experiments orientation experiments. In Section 3, we will spell out in detail the different structural features orientation experiments have. For now, let us confine ourselves to a rough characterization. Orientation experiments are a special type of intervention experiments used to provide evidence for or against a qualitative characterization of a mechanism. They do not go ‘in depth’, actively identifying entities and activities of the mechanism, but instead remain on the level of the explanandum and the environment. As such, they are epistemologically prior to inter-level experiments, which require more detailed knowledge.

We will argue that orientation experiments serve a number of purposes. In particular, they are important in the discovery of mechanisms, because they guide and constrain future, more detailed investigation into the mechanism, and exclude alternative mechanisms. As such, they can be used to settle the competition between rival hypothetical types of mechanisms. To give a very brief example (discussed in more detail below), if we wonder how ants are able to find the shortest way between their nests and a food source, an orientation experiment can tell us that the mechanism is chemical rather than visual. Though abstract, such a characterization is by no means trivial, since it has implications for future research.

We will argue that orientation experiments are typical of the early stages of investigation, when an explanandum-phenomenon has only recently been addressed. There is a stage in between the identification of the explanandum-phenomenon and the discovery of the mechanism, in which orientation experiments are devised and performed. We will call this the orientation-phase. To illustrate this, we will consider three case studies of different eras: the capacity of eels to produce numbing sensations (18th century), cadaverous poison as famously discovered by Semmelweis (19th century), and the capacity of pigeons to home (20th century). Besides illustrating the structural features of orientation experiments, the case study also help us to flesh out the different goals these experiments can serve—goals that set them apart from inter-level experiments.

In order to avoid misunderstanding of our project in this paper, it is important to emphasise that other authors have acknowledged the existence of different types of experiments in the discovery of mechanisms (e.g. Craver & Darden, 2013, chapter 8). Our contribution lies in the fact that we develop philosophical insights about the structure and epistemic role(s) of one of these types, the type that we call orientation experiments.

Let us conclude with an overview. In Section 2, we will first clarify some terminology and sketch the state of the art in the literature about the discovery of mechanisms, both with respect to the different phases of this research and the experimental strategies used during these phases. In particular, we will focus on Craver's (2007) inter-level experiments. In Section 3, we will argue that these experimental techniques presuppose knowledge that is not always present, and that in fact, in between the identification of the explanandum-phenomenon and the constructing of detailed models of the mechanism, lies a phase which we dub the orientation phase, during which what we call orientation experiments are carried out. We spend the remainder of Section 3 characterizing the structural features of this type of experiment. In Sections 4 First case study: electric eels, 5 Second case study: puerperal fever (a.k.a. childbed fever), 6 Third case study: pigeon navigation respectively, we illustrate the points developed in Section 3, and flesh out the different goals orientation experiments serve, by considering the case studies mentioned above. Finally, in Section 7 we describe how our ideas on orientation experiments fit into an overall view on (various types of) experiments and their role in the discovery of mechanisms.

Section snippets

Mechanisms, organization and explanation

A decade and a half have passed since Machamer, Darden and Craver's seminal article (2000) sparking the debate about mechanistic explanations, so by now, most readers will be familiar with the most important concepts involved. Therefore, we shall not rehearse this debate in detail, but simply limit ourselves to giving some working definitions of the concepts involved. We adopt these definitions from Illari & Williamson (2012):

A mechanism for a phenomenon consists of entities and activities

Orientation experiments and the orientation-phase

According to the literature then, the discovery of mechanisms consists of identifying the phenomenon, generating mechanism sketches and then evaluating and revising these sketches by means of inter-level experiments. While we think that all these phases and experimental techniques are valuable, we feel that there is something missing from this picture.

In particular, the kind of inter-level experiments Craver describes, presuppose a detailed knowledge of the mechanism that is not always present.

First case study: electric eels

For our first case study,4 we will consider the investigation in the 17th and 18th centuries into the capacity of certain ‘eels’,5

Second case study: puerperal fever (a.k.a. childbed fever)

Our second case is from the 19th century: the work of Ignaz Semmelweis at the Vienna Maternity Hospital, which was made famous by Carl Hempel in his 1966 book The Philosophy of Natural Science. Our account is mainly based on Loudon, 1992. While Hempel uses the popular term “childbed fever”, the standard (medical and historical) term (used by Loudon and many others) is “puerperal fever”. We use the latter term. This case study will be used as a further illustration of the structural features of

Third case study: pigeon navigation

Our third case study8 is from the 20th century and relates to the capacity of homing pigeons (Columba livia) to home. A trained pigeon can be taken from its home, transported over very long distances (hundreds of miles are not uncommon), in total darkness,9

Experiments and the discovery of mechanisms

In their 2013 book Craver and Darden write that “Interlevel experiments […] are not different in kind from experiments for testing causal relevance, but rather should be viewed as particular kinds of such experiments” (ch. 8, p. 129). We quote this because we endorse this view on interlevel experiments, but also because we think that something analogous holds for orientation experiments. Let us clarify this.

All the experiments we have discussed somehow provide evidence for or against a causal

Conclusion

In this article, we have argued that the literature on the discovery of mechanisms, though it is very detailed in its description of the different strategies used by scientists to construct detailed models of the mechanisms responsible for all kinds of capacities, has neglected an important strategy that is prominent in the discovery of mechanisms: the orientation experiment. These experiments are crucial in the early stages of investigation, in fact constituting a distinct phase that comes

Acknowledgements

The research for this article was supported by the Research Fund Flanders (FWO) through grant nr. G0B0714N. The authors thank their colleagues at Ghent University, especially Leen de Vreese and Dingmar van Eck for comments on previous versions of this article. Parts of this article were presented at the ‘Objectivity in Science’ conference, held in June 2015 at Tilburg University; we extend our thanks to the audience for providing us with valuable comments.

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