Mind Your Own Business
According to Sigmund Freud, projection is a psychological defense mechanism whereby one "projects" one's own undesirable thoughts, motivations, desires, and feelings onto someone else. It is a common process that every person uses to some degree.
To understand the process, consider a person in a couple who has thoughts of infidelity. Instead of dealing with these undesirable thoughts consciously, he or she subconsciously projects these feelings onto the other person, and begins to think that the other has thoughts of infidelity and may be having an affair. In this sense, projection is related to denial, arguably the only defense mechanism that is more primitive than projection. Those who project deny a part of themselves that may otherwise come to the surface. In this case, they cannot face their own feelings of infidelity and therefore project them onto the other person.