In "Think Again," Adam Grant defines rethinking as both a skill set and a mindset essential for navigating a rapidly changing world. He emphasizes that, while we often rely on our existing knowledge and expertise, it is crucial to spend as much time rethinking as we do on forming opinions. Rethinking involves questioning our deeply held beliefs and assumptions, allowing us to adapt our perspectives when new information arises. Grant highlights that this process liberates us from the constraints of outdated views and can lead to a more fulfilling life. He also points out that cognitive laziness can hinder our willingness to rethink, as many people prefer the comfort of old beliefs over the challenges of reconsidering them. Overall, rethinking is about maintaining mental flexibility and having the courage to adjust our views in light of new evidence.