Content Warning: This section of the guide includes discussion of mental illness, substance use, addiction, and death.

ACE is a three-step formula Harris introduces for managing difficult thoughts and feelings. The acronym stands for Acknowledge thoughts and feelings, Connect with the body, and Engage in what one is doing. This technique forms the foundation of Harris’s approach to mindfulness and psychological flexibility. The components can be practiced in any order, but all three elements must be present for the technique to be effective.

Acceptance and Commitment Therapy (ACT) represents a psychological model developed in the mid-1980s by Stephen C. Hayes and his colleagues Kelly Wilson and Kirk Strozan. This therapeutic approach has generated over 3,000 published studies demonstrating its effectiveness in treating various conditions, from depression and anxiety to chronic pain and trauma. ACT practitioners operate in numerous countries and settings, including medical facilities, schools, government agencies, and professional sports teams. The model focuses on helping individuals build meaningful lives while accepting the presence of difficult emotions rather than attempting to eliminate psychological discomfort. Though initially created as a therapeutic intervention, ACT has evolved into a broader framework for enhancing general well-being, improving performance, and increasing resilience.