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Signals are minimal causes

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Abstract

Although the definition of ‘signal’ has been controversial for some time within the life sciences, current approaches seem to be converging toward a common analysis. This powerful framework can satisfactorily accommodate many cases of signaling and captures some of its main features. This paper argues, however, that there is a central feature of signals that so far has been largely overlooked: its special causal role. More precisely, I argue that a distinctive feature of signals is that they are minimal causes. I explain this notion, suggest some strategies for identifying its instances and defend its relevance by means of conceptual and empirical considerations.

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Notes

  1. In most of the examples we will discuss, the roles of sender and receiver are played by different organisms, but they can also be parts of the same organism (e.g. brain states, hormones).

  2. Probably, processes other than natural selection (such as learning) can also ground functions (Abrams 2005; Frick et al. 2019; Shea 2018). Nonetheless, for simplicity, and given that most of the examples we will discuss involve evolutionary functions, I will focus on those.

  3. Nonetheless, not everyone agrees with this picture. For instance, some people hold that an evolved sender is not required (e.g. Stegmann 2009; cf. Artiga 2014). Fortunately, I think that nothing I'll say here depends on this claim. Others, however, oppose the idea that animal communication involves an exchange of meaningful signals (Dawkins and Krebs 1978; Rendall et al. 2009; Rendall and Owren 2013). This latter approach will be briefly discussed in Sect. 4.3.

  4. In certain special circumstances, saliva might actually work as a signal. For instance, some stingless bees use saliva to lay a trail that indicates that an important source of food has been found (Schorkopf et al. 2007). Nonetheless, it is much harder to accept that saliva plays a representational role simply in virtue of how it is used in digestion.

  5. Millikan (1984) was one of the first philosophers to propose a teleological theory of representations along the lines of RV.

  6. As a reviewer suggested, one might object that it is not obvious that the mouth or the stomach has the function of responding to the presence of saliva. In any case, I think this example is not only of historical interest, but also points in the direction of a range of similar examples that do seem to raise a real challenge.

  7. Nonetheless, I think that some objections along the same lines actually fail to challenge RV (Scarantino 2013a: p. 76; Scott-Phillips 2008). RV, for instance, rightly excludes reciprocal interactions from being signals. Consider grooming: it alters the behavior of another individual (toward reciprocity) and probably evolved because of this effect, but in this case there is no clear condition C that grooming is supposed to correspond to, and whose presence causes the recipient's action to increase fitness. Certainly, sometimes grooming works as a signal, as when a chimpanzee engages in this activity in order to indicate her willingness to build an enduring relationship (de Waal 1982). However, in this case, the sender’s positive attitude toward engaging in a relationship plays the role of C, so RV correctly entails that it is a signal (cf. Kalkman 2019).

  8. These two conditions could be expressed more succinctly as follows: ‘S is a non-enabling cause of A.’ Nonetheless, for clarity of exposition, I prefer to keep these two claims separate here.

  9. 'Intervention' is here a technical term: it roughly means an ideal manipulation by which we change the value of a particular variable.

  10. Another criterion provided by Macedonia and Evans (1993: p. 179), which is still very important in actual research (Shettleworth 2010: p. 515) is production specificity: a signal that refers to a particular object must reliably be given in its presence, and not under other conditions (see also Smith 1991: p. 215). Although this criterion might be heuristically useful for identifying signals, I presume it probably fails to point to a distinctive property of signals. In short, my worry is that if this criterion is understood strongly, then it is incompatible with the possibility of misrepresentation (i.e. the fact that signals are sometimes given in the wrong conditions; see Scarantino 2013b), whereas weakly understood it just claims that signals carry correlational information of whatever they represent, which probably follows from RV above, and does not seem to distinguish signals from other causes.

  11. This suggests that the notion of a ‘signal’ is vague, in the sense that it has borderline cases. In general, I think we should expect that to be true for most biological categories (Godfrey-Smith 2009).

  12. As a reviewer suggested, the notion of a ‘nearly complete’ mechanism plays an important role here. I explained above some aspects that are relevant for understanding this notion (e.g. it comes in degrees, some aspects might be more important than others,…), and I hope that future work will allow me to specify this notion in more detail.

  13. These experiments are also used to draw conclusions about the cognitive mechanisms involved in signal perception (i.e. to investigate whether it involves cognitive representations or not; see e.g. Kalkman 2017; Zuberbühler 1999).

  14. I will interpret ‘information’ as ‘semantic information’ (i.e. semantic content), not as Shannon information.

  15. An additional reason for thinking that this analysis challenges the traditional attribution of informational content to the túngara’s frog’s signals is that, on this analysis, it is not obvious that the receiver has a signal-specific response. In any event, my goal here is to provide one reason why the manipulationist framework is in tension with an informational one, but this is compatible with other ways of framing the debate. I would like to thank a reviewer for pressing me on this issue.

  16. It should be mentioned that there are certain formal properties of the signal that can explain behaviour without compromising its status as a minimal cause. For instance, when complex signals exhibit some form of systematicity or compositionality, the action might depend on some formal properties. I hope the discussion in the main text illustrates the kind of intrinsic properties that are in tension with the state’s being a minimal cause (e.g. its being an enabling cause).

  17. Interestingly, those who endorse a representational/informational paradigm reply that their approach enables theorists to apply the same mathematical models across taxa and modalities, whereas manipulationists hold that this position has contributed to neglecting the study of signal design (Seyfarth et al. 2010; Rendall et al. 2009; Stegmann 2013: p. 21). This is precisely what we should expect if we assume that signals are minimal causes: to the extent that a signal is representational, its format is much less explanatory. For this reason, a representational/informational perspective facilitates a more widespread use of similar models and tends to pay less attention to the signal’s formal properties.

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Acknowledgements

I would like to thank Leonardo Bich, Mihnea Capraru, Marta Campdelacreu, Sabrina Engesser, Samir Okasha, Javier González de Prado Salas, Kirsty Graham, John Horden, Manolo Martínez, Richard Moore, Saúl Pérez, Aida Roige, Miguel Ángel Sebastián, Nicholas Shea, Dan Zeman and the audiences at the VII SEFA Conference, the Seminario de Filosofía de la Mente at the Instituto de Investigaciones Filosóficas at the Universidad Nacional Autónoma de México, the ISCHPSSB 2019 Conference at the University of Oslo and the 1st Varieties of Information Workshop on Animal Communication at the University of Barcelona. Financial support was provided by the Ministerio de Economía y Competitivdad through the projects ‘La Complejidad de la Percepción: un enfoque multidimensional’ (FFI2014-51811-P) and ‘Varieties of Information’ (PGC2018-101425-B-I00).

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  1. Departament de Filosofia, Facultat de Filosofia i Ciències de L’Educació, Universitat de València, Av. Blasco Ibàñez 20, 46010, Valencia, Spain

    Marc Artiga

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Artiga, M. Signals are minimal causes. Synthese 198, 8581–8599 (2021). https://doi.org/10.1007/s11229-020-02589-0

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