The Principle of Similarity[]
The principle of similarity, also known as the "association of ideas", which falls under the category of sympathetic magic, is the thought that if a certain result follows a certain action, then that action must be responsible for the result. Therefore, if one is to perform this action again, the same result can again be expected. One classic example of this mode of thought is that of the rooster and the sunrise. When a rooster crows, it is a response to the rising of the sun. Based on sympathetic magic, one might interpret these series of events differently. The law of similarity would suggest that since the sunrise follows the crowing of the rooster, the rooster must have caused the sun to rise. Causality is inferred where it might not otherwise have been. Therefore, a practitioner might believe that if he is able to cause the rooster to crow, he will be able to control the timing of the sunrise. Another use of the principle of similarity is the construction and manipulation of representations of some target to be affected (e.g. voodoo dolls), believed to bring about a corresponding effect on the target (e.g. breaking a limb of a doll will bring about an injury in the corresponding limb of someone depicted by the doll).
Explanation of the Principle of Similarity[]
If an action is associated with a re-action then that means that if you could control that action you could then also control the re-action.
( If "a" causes "b" then I can use "a" to control "b" )
Something that represents a target can be enchanted with a spell in order to enchant the target without the target being present.
( If "a" represents or resembles "b" then whatever I do to "a" happens to "b" )
Correspondence in Sympathetic Magic[]
Correspondence is based on the idea that one can influence something based on its relationship or resemblance to another thing. Many popular beliefs regarding properties of plants, fruits and vegetables have evolved in the folk-medicine of different societies owing to sympathetic magic. This include beliefs that certain herbs with yellow sap can cure jaundice, that walnuts could strengthen the brain because of the nuts' resemblance to brain, that red beet-juice is good for the blood, that phallic-shaped roots will cure male impotence, etc.
Many traditional societies believed that an effect on one object can cause an analogous effect on another object, without an apparent causal link between the two objects. For instance, many folktales feature a villain whose "life" exists in another object, and who can only be killed if that other object is destroyed. (Examples including Sauron's One Ring in The Lord of the Rings trilogy, and the Russian folktale of Koschei the Deathless. Compare Horcrux and lich.)
Sources[]
http://en.wikipedia.org/wiki/Sympathetic_magic
http://en.wikipedia.org/wiki/Magic_(paranormal)#Principle_of_similarity