The problem of biological memory led Russell to propose the existence of mnemic causation, a mechanism by which past experience influences current thought directly, that is, without the need for a material intermediary in the form of a neural "memory trace." Russell appears to have been inspired by German biologist Richard Semon's concept of mnemic homophony, which conveys memory to consciousness on the basis of similarity between current and past circumstances. Semon, however, in no way denied a role (...) for stable neural structures or "engrams" in the memory process. In contrast to Russell, contemporary biologist Rupert Sheldrake provides a viable theory of memory by remaining true to Semon's original idea and expanding it beyond personal memory to the collective memory of species, encompassing not only mental but developmental memory. (shrink)