A STORY OF ROBINSON CRUSOE

This »manga« (Japanese word for "comic strip") is based on "A Story of Robinson Crusoe" which Gesell wrote in 1920.
Silvio Gesell (1862-1930) was a businessman who worried about the misery that was caused by economic crises and in which wide circles got caught. He recognized the dysfunctional flaw in the monetary system itself and wrote a number of books on the topic.
"A Story of Robinson Crusoe" is one of his better stories to make his teaching come alive. 

When in our existing monetary system somebody lends out money he will collect interest. Somebody who borrows money has to pay interest.

No one has ever questioned this system.
But is it really so natural?
What is this naturalness based upon?