Family systems therapy is based on the idea that individuals are best understood through assessing the entire family. It suggests that families are systems of interconnected and interdependent individuals, with each having his/her own role to play. Individuals cannot be fully understood in isolation from one another or from the family system, because the family is an emotional unit. It would be like trying to figure out a single puzzle piece without seeing the whole puzzle, or the role that piece has in the whole picture.
Individuals may carry a symptom for the entire family, because symptoms in individuals are often seen as expressions of dysfunctions in the family unit. As a result, a change in one member affects all members. However, individuals can have symptoms existing independently from the family members, but these symptoms always have ramifications for family members. Therefore, family therapists will change the system in order to change the individuals. They do so by changing dysfunctional patterns of relating, and help to create functional ways of interacting.