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On the Causation and Constitution of the Immunological HolobiontIn evolutionary biology, developmental biology, and the cognitive and linguistic sciences, many classical debates are disagreements between “internalist” and “externalist” positions. “Intermediate” positions on this continuum are widely divergent forms of “interactionisms” (Oyama 2001). Philosophers argue that these positions exhibit fundamental disagreements over the explanatory primacy of internal vs. external factors, the location of loci of control, the relative richness and complexity of internal vs. external structures, and/or the spatial grounds/supervenient bases/realizers of higher-level phenomena (Godfrey-Smith 1996; Wilson 2004, 2005; Bechtel and Richardson 2010; Hurley 2010). In these overviews, however, two main issues are not always properly distinguished. The first issue concerns the units of relevant causes: in the complex network of causal interactions, where is the boundary that encloses the causes that are relevant? Externalists, for instance, argue that the causal boundary drawn by internalists are too exclusive and leave out important external factors. The second issue is about the biological individuals in these sciences: what is the nature of these individuals and their physical boundaries? An externalist concerned with this issue, as an example, might argue that the identity and individuation of biological individuals is determined by external factors. Entanglement between these two issues in the other sciences are evident when philosophers argue that we should take a bottom-up approach and let the best causal models determine whether a biological individual should extend to incorporate external factors (e.g. Hurley 2010), or when the boundary of individuals are adjudicated by the success of causal parity arguments between factors internal and external to the individual, or the causal intensity of interactions within individuals vs. between individuals and the world (e.g. Clark and Chalmers 1998, Adams and Aizawa 2008). But the two issues are distinct. The Causal-Constitution Fallacy Argument in both philosophy of cognition and biology question whether expansions of causal networks and interactions beyond the boundaries of the biological individual would entail an expansion of the individual itself (see Menary 2010, Huneman 2013). So far, none of the overviews of debate across the sciences have covered the rich history of internalist vs. externalist debates in immunology. I argue that in the immunological debates as opposed to the others, there is a sharper distinction between the causal sources of immunity vs. the individual that is protected by immunity. While there is disagreement over the internal vs. external causes of immunity, the consensus is that the individual is endogenously defined. Based on the Equilibrium Model of Immunity (Eberl 2010, 2016), I then develop criteria that differentiates between an interactionist position of causality versus an interactionist position of individuality. These criteria help move forward the individuality problem of the “holobiont,” the multicellular eukaryotic host and its persisting residential micro-organisms, by separating arguments on the nature of the holobiont as a biological individual from arguments on the interacting causes that contribute to development and functioning of the holobiont. In sum, I argue for a distinction between units of causality versus individuality by situating immunology within the broad spectrum of internalism vs. externalism debates.
University of Bordeaux/CNRS