 We note, however, that the sheer excitement and novelty of unexpected findings can also lead to exaggerated views of the relationship between human hosts and their microbes. Exactly how responsible microbiomes are for human experiences such as anxiety, depression, learning and memory is uncertain and at least sometimes dubious (for discussion see Hooks et al. 2019). A similarly inflated claim is that knowledge of microbiomes necessitates a new view of self, or of what it means to be human (e.g., Rees et al. 2018; Ironstone 2018). While dozens of microbiome authors have made claims to this effect, with some attempting to explain what it actually means for the microbiome to challenge or change our sense of self (e.g., Rees et al. 2018), this claim still needs a great deal of clarification and substantiation (Parke et al. 2018). A vast range of things could be meant by “self” or “what it means to be human” in the context of these microbiome claims, ranging from something controversial about the nature of human biological individuality (see discussion above), to something factual about the composition of human bodies (i.e., people used to think their bodies were made of only human cells but now know they are made of microbial cells as well), to something phenomenological about first-person human experience (to date, there is no evidence that microbes influence this). So, these claims about the influence of microbiomes on “who we are” are at best speculative and exaggerated. Some of these conceptual problems probably come about because of unclarity about how to investigate the causal powers of microbiomes, and more generally about causal explanation in microbiome research.