A walnut looks like a brain, and therefore could perhaps cure brain diseases — today, a traditional medicinal idea called ‘the doctrine of signatures’ is widely cited in contemporary pharmacy and herbalism. It holds that many minerals, plants and animals have ‘signatures’, i.e. visible resemblance to human organs, bodily fluids or disease symptoms, and that these signatures indicate the curative effects of natural things. Throughout the centuries, the doctrine of signatures has been veiled by layers of constructions: some argue that it originated from sixteenth-century Swiss physician Paracelsus, some trace it back to antiquity, and some see it as a...