Explanation:
To start with, one must understand what Albert Einstein meant when he said: "Imagination is more important than knowledge. Knowledge is limited. Imagination encircles the world." Albert Einstein (1879-1955).
In "I have a Dream" the songwriters have properly divided the world into two main spheres, the real world, and the supernatural world where imagination and fantasy dwell with such beings as angels, who only become "real" to us in our dreams.
In this song, these two worlds are divided by an imaginary "stream", but it is a stream that one can choose to cross at will, going from the real world into one's spiritual world, or into one's imagination "whenever the time is right to be in that sphere".
