Caitlin ran to her mother to tell on her brother, Sam. Caitlin said that Sam hurt her hand when they passed each other on the stairwell. Understanding conflict, Caitlin's mom decided to sit down with both of the children and ask them what happened, starting from when they saw each other on the stairs. She found out that Caitlin, in fact, hurt her hand when she hit Sam. Sam did nothing in return! Which principle of the system theory is Caitlin's mom recognize?