This law assumes that one can influence something based on its relationship or resemblance to another thing. It's the magical law that describes Sympathetic Magic and also influences many other forms of magical workings. It can be further broken down into two counterparts: The Law of Contagion, and The Law of Similarity. 

Like the magical Law of Contagion, The Law of Similarity governs Sympathetic Magic. This law states that a Magician can produce any effect by simply imitating it. In other words, having a physical or mental representation of a thing facilitates the ability to have control or influence over said thing. 
This law supports magical workings like voodoo dolls, poppets, figure candles, and all other spells that involve assigning representation to an object to effect the thing/person it represents.

First introduced by George James Frazer in The Golden Bough, the law of contagion states that once two objects/people have been in contact, a magical link persists unless/until a magical act (such as cleansing, consecration, exorcism, cord cutting, or other act of banishing) breaks the non-material bond. This law is one of the two fundamental laws that Sympathetic magic is founded upon. 
This is a powerful law (with parallels in Quantum Physics according to Isaac Bonewits) that holds true for non-magic believers (read: muggles) as well. For example, a person who does not believe in magic or believe themselves to be 'superstitious' might still have reservations about living in a house where murders took place, or owning an item previously belonging to a person deemed to be 'evil'.