This lecture presents the idea, history, and logic behind the doctrine of signature. Part of all planetary cultures, traceable in Western history since the ancient Egypts, and coming down to us via Paracelsus and the medieval great herbal masters, the doctrine of signatures is the most artistic and creative part of herbalism. Learning to read the language of plants spelled out to us in their colors, shapes, smells, tastes, and environment, we can start to read in the book of Nature.