Justin E. H. Smith at his own website:
Mainstream philosophy and science at least since Aristotle have held to the view that each living body, under normal circumstances, should be inhabited by no more than one soul. If you don’t care for talk of souls, exchange that word for “individual”, and the point still stands: sharing bodily space is abnormal, a sign of pathology, to be corrected by flushing the worms out of your entrails, or by mulesing your sheep against flystrike.
It was this prejudice against mutualism that delayed by several years widespread recognition of the true nature of lichen: not a moss, not some low liverwort, but an intrication of two very different kinds of being, of fungus and algae, the former hosting the latter, as near as we can make out, farming it if you will for the miraculous calories it photosynthesizes out of pure light.
We continue to act surprised even though in truth this is the regular course of things. The human microbiome is home to over 5000 known species of microorganism, without which many basic bodily functions, notably digestion, could not continue. The baleen of a whale’s mouth constitutes a marine ecosystem more comparable to the biodiversity of a kelp forest than to a single animal’s body part. Biology furnishes us with abundant empirical examples of a truth that logic and metaphysics, in the form we have inherited them, still require us to reject on a priori grounds. If you do not wish to appear irrational, do not learn too well the lessons of natural science.