The psychologist, Eckhard Hess, described the pupils as windows to the soul: their magnitude of dilation is a signal of mental effort. During a cognitive task, such as multiplication, the pupil dilates to a large size within a few seconds and remains large until the person either finds a solution or gives up. As you become skilled in a task, the demand for this mental energy diminishes, as scientists can see this in a reduction in pupil dilation. Focused, slow thinking is effortful - we often operate more on instinct than by stopping and working through every action or decision. A general “law of least effort” applies to cognitive exertion, as it does to physical. When there are several ways to achieve a goal, people will gravitate to the least demanding.