What prompted the happy medium to say, “it can be overcome,”?

The Happy Medium saw the light by the Black Thing overcoming it.

awnser:

jem is "moody and silent" after the pants incident. the new school year starts, and scout finds second grade just as boring as first. one day, she and jem are walking home together when jem reveals that when he found his pants that night, they were all folded up, and the tears had been crudely sewn up, as if someone knew that he would be coming back for them. he finds this highly eerie. then, they find a ball of twine in the radley oak tree knothole. again, they aren't sure if it is a gift for them or not, so they leave it for a few days. when it remains in the hole for a few days, they take it, and decide that anything left there is okay to take.