How does the author use foreshadowing in The Doors of Perception, and Heaven and Hell?

The ego, according to psychologist Sigmund Freud, is the part of personality that controls decisions and thinks realistically about what has to be done. Without the function of this part of the personality, mescalin users are not interested in daily activities and are more interested in experiences and sensations. This explanation of how mescalin works is important for the essay as a whole because it foreshadows some of the key pros and cons that Huxley will later describe in regard to whether mescalin should be used when one wants to have a visionary experience.