Doctrine of signatures

Leading people to believe that they can treat depression by eating walnuts because they look like brains is dangerous and irresponsible, just like saying homeopathy can treat, oh I don’t know, anything at all. These are dead theories, and their rotting corpses must be buried to keep them from stinking up the place. The doctrine of signatures was a concept developed in ancient Europe that claimed that plants advertised their usefulness to humans and animals by looking like the things they aided. In addition to the plants looking like human body parts, animals or other objects, the doctrine of signatures could also refer to the environments or specific sites in which plants grew.

Christianity
Late medieval Christian theologians embraced the concept as a natural extension of the idea that the Creator carefully ordered his creation such that careful observation would reveal useful information about the plant. Walnuts that looked like the cranium would be used for headaches, or beans that looked like livers would cure liver problems. Beyond medical applications theologians extended this concept to plants representing all the right doctrines of Christianity. This is most aptly seen in the Passionflower (Passiflora incarnata) whose symmetry and numerical elements were said to be representative of Jesus Christ's passion.

Mandrake (Mandragora spp.), whose roots sometimes contain bifurcations causing them to resemble human figures showing off their legs, in Hebrew is "love plant". There are classical Jewish commentaries which suggest that mandrakes help barren women to conceive a child.

Homeopathy
Homeopathy has often called upon the doctrine of signatures for finding potential herbs to use in their cures. Despite its complete rejection as a valid theory for classification of plants or their properties and uses in medicine, many homeopathic and other "natural" practitioners still use the concept today.

Examples
Some examples of plants and their "uses" based on the doctrine of signatures:
 * Lousewort (Pedicularis spp.) &mdash; thought to be useful in repelling lice
 * Spleenwort (Asplenium spp.) &mdash; thought to be useful in treating the spleen
 * Liverwort &mdash; thought to be useful in treating the liver
 * Toothwort (formerly Dentaria spp., now Cardamine spp.) &mdash; thought to be useful in treating tooth ailments
 * Hedge woundwort (Stachys sylvatica) &mdash; has antiseptic qualities
 * Lungwort (Pulmonaria spp.) &mdash; used to treat pulmonary infections
 * Wolfsbane (Arnica montana or Aconitum spp.) &mdash; used to treat Lycanthropy