From an anthropological perspective, analogy is a form of thought that posits that things, people and events are reflected in each other. For analogical thinking, knowing is deciphering similarities; analogy unveils a world of secret links underlying reality, and generates a “cosmic feeling where triumph order, symmetry, perfection”, a closed world (Gadoffre & al. 1980, p. 50); thus conceived, analogy is the foundation of gnosis. From the perspective of the history of ideas, this form of thinking culminated in the Renaissance, when our “sublunary » world was, by analogy, mapped with the heavenly spheres, and with the divine world more generally.

In one of its manifestations, the doctrine of analogical correspondences validates the following type of argument:

Data: This plant looks like such or such part of the human body.
Conclusion: This plant has a hidden virtue, effective to cure the ills that affect the corresponding part of the body.
Warrant: If the shape of a plant is like a body part, then it cures ailments affecting that body part.
Backing: This is a divine provision.

This form of analogical thinking postulates that plants have hidden medicinal properties. The plant bears a divine signature, that is, a representation of the human body part that it can heal. This signature or “analogical sympathy” is a motivated signifier, a similarity or “resemblance” of the given body part. God, in his benevolence, has imposed this signature on particular plants in order to make them of use to us. A plant resembling the eyes, therefore might cure eye irritation.

Since the skin of the quince is covered with small hairs, it bears the “signature” of the hair, and eating the quince can make your hair grow. In the wording of Oswald Crollius [1609]:

Data: ‘This downy hair growing around quinces […] represents hair in some way.” (id., p. 41)
Conclusion: “So, their decoction makes hair grow, which fell because of the pox or another similar illness.” (ibid.)
Warrant: the healing power of plants “can be recognized more easily by the signature or analogical and mutual sympathy with the members of the human body with these plants than by anything else.” (id., p. 8)
Backing: “God gave an interpreter to each plant so that its natural virtue (but hidden in its silence) can be recognized and discovered. This interpreter can be nothing else than an external signature, that is to say a resemblance of form and figure, true indications of the goodness, essence and perfection thereof.” (id., p. 23)
Oswald Crollius, [Treatise on Signatures, or the True and Living Anatomy of the Big and the Small World]; [1609][1]

From this doctrine derives a research program for “those who want to acquire the true and perfect science of medicine”, “they should devote all their efforts to the knowledge of signatures, hieroglyphs and characters” (id., p. 20). Training will enable them to recognize “at first glance, on the surface of the plants, what faculties they are endowed with” (id., p. 9).

The knowledge of the medicinal properties of plants is acquired by learning how to read and understand the “discourse of nature”, that is to say, by mastering the signs scattered around the world. Such an analogical reading of the world is opposed to empirical causal investigation, which consists of observation and experience, practicing dissection or prescribing a concoction to the patient and then finding out if he or she is better, dead, or neither better nor worse. Analogical knowledge is a specific mode of thought, constitutive of magical thinking that substitutes for causal knowledge mysterious correspondences conveying influences, and bypasses the hierarchical system of categories organized according to genus and species, for which it substitutes a similarity network.

[1] Quoted after Oswald Crollius, Traicté des Signatures ou Vraye et Vive Anatomie du Grand et Petit Monde. Milan: Archè, 1976.