Just throw up the building without much preparation. A foundation of rock may require some chiseling, or moving the heavy rocks around. Sand is shaped much more easily. The foundation involves the base of our lives. It is a philosophy of life or a guiding principle.
Different people build on different things.
For many Americans the foundation seems to be career. We begin to prepare for it in college - we move toward that goal. We think we will get financial benefits. Then we can get the things we enjoy in this life. I have watched storms take career right away.