55 - Internal Models are models that we build up inside our minds over time. They can represent your natural understanding of a subject matter such as the laws of physics or the steps in making the perfect cup of espresso. They are the ones that have become so well ingrained that you stop thinking about them and they become habits.
Internal Models can also take the form of explicit tools that you’ve memorized to help with thinking about problems. Typically internal models start out as explicit ones and naturally turn into implicit ones with repeated use. This is why intuition exists, and is commonly experienced by experts.