The Doctrine of Signatures affirms that like objects influence one another, and that the appearance and habits of certain plants reveals their medicinal and magical uses. Hence a walnut, appearing similar to a brain, may be used for issues of cognition and memory.
It is a belief stretching back nearly two millennium, and witnessed in the writings of herbalists such as Dioscurides and Galen, and later developed by Paracelsus and Bohme. For such herbalists this belief supposed the hand of a Creator in nature, who through His design gave us signs as to what certain herbs may be used for. This emphasis on God helped place herbal remedies under the authority of religion, which had at times considered the practice of medicine to be against the natural order and therefore ungodly.
In "The Order of Things" the philosopher Michel Foucault writes: "Up to the end of the sixteenth century, resemblance played a constructive role in the knowledge of Western culture. It was resemblance that largely guided exegesis and the interpretation of texts; it was resemblance that organized the play of symbols, made possible knowledge of things visible and invisible, and controlled the art of representing them."