Mansion

The speaker believes as one ages, the mind grows into a mansion, or storage tank, in which one can store all these great memories in life, and refer back to them. As an adult, these memories are a source for moral behavior. This transition from childhood to adulthood is "From joy to joy" (125), and he tells his sister that it will help her to overcome the evil present in the world. This stresses how his thought process has changed since his youth.