The human brain operates using two distinct cognitive systems: the Automatic System and the Reflective System. The Automatic System is fast, intuitive, and often unconscious. It's responsible for our instinctive reactions, such as ducking when a ball is thrown at us unexpectedly or smiling when we see a cute puppy. This system is associated with the oldest parts of the brain, the parts we share with lizards and puppies.

On the other hand, the Reflective System is slow, deliberate, and self-aware. We use this system when we are faced with complex tasks that require conscious thought, such as calculating a difficult math problem or deciding which route to take for a trip to an unfamiliar place.

These two systems help explain the paradox of human intelligence, where we can be ingenious in some tasks and clueless in others. For instance, accomplished athletes can rely on their Automatic Systems to perform complex tasks with amazing accuracy and exceptional speed, thanks to countless hours of practice. However, they might struggle with simple tasks that require the Reflective System, such as calculating a difficult math problem.

A story that illustrates the interaction between these two systems involves a nine-year-old boy named Declan, who was unable to resist toy stores. His Automatic System would urge him to go inside and buy something, even though his Reflective System knew he had enough toys. This story highlights how our Automatic System can sometimes override our Reflective System, leading us to make decisions that may not be in our best interest. This is where the concept of "nudging" becomes particularly relevant. "Nudging" is a concept in behavioral science that involves subtly guiding individuals towards beneficial decisions without restricting their freedom of choice. It is designed to take advantage of the Automatic System's tendency to respond to environmental cues and guide it towards better choices.