The working concepts that have emerged in microbiome research bear an uncanny resemblance to one of the most ancient traditions of Western medicine: humoural medicine as promulgated by the medical practitioner and philosopher Galen in the second century CE. In particular, both Galenic medicine and medical microbiome research rely heavily on notions of imbalance and balance, with undesirable unbalanced states called ‘dyskrasia’ and ‘dysbiosis’ respectively. Therapeutically, both systems aim at restoration to a balanced state. Both traditions also hold that the composition of the focal entities (humours or microbiomes) determines not just every bodily state but mental ones too. Causality for each is conceived teleologically, meaning that parts of bodies ‘function for’ the maintenance of the whole. And ultimately, each framework asserts that external environments are part of the balance equation, thereby situating the humours or microbiomes in a unified multilevel theory that purportedly explains the very nature of health and perhaps even humans. As well as describing the parallels between these systems, we seek to explain them: Should we think of these resemblances as due to direct historical continuity, or due to incidental convergence? Finally, we address the implications of these abundant similarities. Should medical microbiome researchers be concerned that their field currently shares conceptual parallels with Galenic medicine, or is it something to celebrate? Ultimately, this is an evaluation all medical microbiome researchers will need to make for the future of their field
