Principle of Contagion[]
This principle suggests that once two objects come into contact with each other, they will continue to affect each other even after the contact between them has been broken.
This means that you can cast a spell on an object that once belonged or belongs to someone in order to effect that person.
( If "a" owns or once owned "b" whatever I do to be "b" will affect "a" )
This also means that if a person has a spell on them contact between that person and another object then enchants that object with the spell.
( If "a" is enchanted with a spell/curse and gets in contact with "b" then "b" will become enchanted with that same spell/curse )
Sources[]
http://en.wikipedia.org/wiki/Law_of_contagion
http://en.wikipedia.org/wiki/Magic_(paranormal)#Principle_of_similarity