We can deduce that the initial reason that Arthur Radley's father shut him inside was because:
Arthur's father considered the outside world to be too evil and wanted to shield his son from any temptations.
This question is related to "To Kill a Mockingbird".
"To Kill a Mockingbird" refers to a novel that was written by Harper Lee. It's a story that reveals the roots and consequences of racism.
Boo was actually locked away by his father in order to shield his son from the wrong crowd. His father saw that the outside world is too evil for his son. Boo was caught with the Cunninghams from Old Sarum, the nearest thing to a gang ever seen in Maycomb.
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