Polyps posed a problem for the eighteenth-century soul. When Abraham Trembley in the summer of 1739 dissected these little pond creatures into increasingly smaller fragments, he observed that each piece could regenerate itself into an entire polyp. He even turned one inside out and watched it continue to live and propagate. But if these little bits of water hydra could grow back into whole polyps, then where was its soul? Did it not prove, as Diderot pointed out, that there was in fact no such thing as a soul; the property of life was simply scattered across all matter (plate 5).

In his Allgemeine Geschichte der Musik (1788–1801) Johann Nikolaus Forkel tried to resolve the problem of the soul, but he must have read about the green-arm polyps that Johann Friedrich Blumenbach had dissected; these polyps, ‘although amply fed, were always smaller than the [original]’. ‘A mutilated rump’, Blumenbach continues, ‘always diminished in proportion very evidently, and seemed to become shorter and thinner, as it generated the lost parts.’ The dissemination of the soul in these aquatic creatures was therefore also a dilution of their being; a secondary polyp could never reflect the fullness of the original. If life is to be modelled on the polyp, then the identity of the human race is distributed throughout the world in mutilated forms, leaving its soul bereft of its original potential. This was a problem. Forkel's solution was simple; he basically reversed the dissecting process to reformulate the soul.