[I invited Pierrick Bourrat to respond to my two posts about his new paper and to comments to those posts. He kindly agreed, and he provided the following guest post, which I have edited only for formatting.]

First of all, I would like to thank Matthew Herron for his interest in my work and his invitation to respond to his posts. Also, I would like to thank Rick Michod and Deborah Shelton for their comments.

I will respond to several issues pointed out both in the posts and the comments.

About the usefulness of the export of fitness view of ETI: I agree that it is a useful way of thinking about it, as long as it is used as a heuristic. This means that I am not inclined to think that building models with the assumption that the fitness of a cell would have been 0 had it been in an environment with not social partners will be able to explain in some deep sense ETIs (and even more so the origin of fitness at some level). In his comment to Matthew’s first post, Rick Michod claims that I somehow confuse realized fitness from a more counterfactual notion of fitness.  Well, to be honest, I do not see how one could simulate (I do not mean ‘explain’) the evolution of a process if the variables in the model do not correspond to realized properties of the system. If I want to model a particular phenomenon, I ought to use variables and parameters that represent the target system and clearly, at least for me, this counterfactual notion of fitness does not represent any properties the cells have because they always have social partners. It is common to use expected rather than realized fitness in models, but this assumption is justified when we can assume that population are large and the environment is overall not fluctuating too much. With the counterfactual notion of fitness, aside from being useful for explaining the ETIs, I fail to see how it could be successfully integrated in models (by successfully, I mean how it could represent meaningfully the target system).

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