When Dr. Milton Erickson was asked by one of his students what he believed to be the foundation of most people's problems, his response was a single word: "Rigidity." This answer encapsulates the significance of psychological flexibility, which is one of the key drivers of positive change. Rigidity is an inflexible pattern of thought and behavior that prevents appropriate responses to the changes and challenges that arise (Leifer, 2003). A person who suffers from rigidity struggles to adapt to variables, adhering to the same response patterns in most situations of their life. It can be said that rigidity manifests as mental, behavioral, and emotional reactions that remain unchanged despite shifting circumstances and contexts. The primary goal of this book is to guide and educate individuals psychologically.