Miss Strangeworth and Mrs. Harper know each other fairly well. They belong to the same bridge club and call each other by their first names, as they do when they happen to meet in the grocery store earlier. Miss Strangeworth's first name is Adela and Mrs. Harper's first name is Martha. Adela Strangeworth has already sent several of her poison-pen letters to this poor woman who appears to be someone she has known for years. That day when Miss Strangeworth gets home she writes another.

After thinking for a minute, she decided that she would like to write another letter, perhaps to go to Mrs. Harper, to follow up the ones she had already mailed. She selected a green sheet this time and wrote quickly: Have you found out yet what they were all laughing about after you left the bridge club on Thursday? Or is the wife really the last one to know?

She is obviously suggesting that Martha Harper's husband has been having an affair with some women in the town and that everybody knows about it except his wife. This was probably more or less what Adela Strangeworth had hinted at in "the ones she had already mailed." Why is she doing this? She must be crazy. She doesn't realize that she is being cruel. From her point of view she is just warning Mrs. Harper of the "possibility" that her husband could be having an extramarital affair. When she runs into Martha Harper at the grocery store earlier, she notices that the woman seems different.

“Ran out of sugar for my cake frosting,” Mrs. Harper explained. Her hand shook slightly as she opened her pocketbook. Miss Strangeworth wondered, glancing at her quickly, if she had been taking proper care of herself. Martha Harper was not as young as she used to be, Miss Strangeworth thought. She probably could use a good, strong tonic… 

Miss Strangeworth is keenly observant. She takes an interest in all the people in "her town." Yet she doesn't realize that her anonymous letters to Martha Harper could be creating nervous problems that cause her hand to shake. Miss Strangeworth notices other people who seem troubled too: the grocer, for instance:

Mr. Lewis looked worried, she thought, and for a minute she hesitated, but then she decided that he surely could not be worried over the strawberries. He looked very tired indeed….

We learn later that she has sent him at least one anonymous letter in which she suggested that his grandson might be stealing money out of the cash register.

Many people seemed disturbed recently, Miss Strangeworth thought. Only yesterday the Stewarts’ fifteen-year-old Linda had run crying down her own front walk and all the way to school, not caring who saw her. People around town thought she might have had a fight with the Harris boy, but they showed up together at the soda shop after school as usual, both of them looking grim and bleak….

Linda Stewart and Dave Harris are two more victims of Miss Strangeworth's letters. She has written to Linda's parents hinting that the two young people have been going far beyond the usual hugging and kissing stage of adolescent romance. Miss Strangeworth does not understand her own motives. She seems to be a lonely spinster who is jealous of anybody who has anyone to love. This is apparently the case with Martha Harper and her husband. She is planting suspicions in Mrs. Harper's mind--not only of her husband but of all the women in the town with whom he could be having an illicit affair.

Miss Strangeworth never concerned herself with facts; her letters all dealt with the more negotiable stuff of suspicion.

This sweet little old lady does not realize the possibility of evil that exists in herself.

What can you infer from the way that Mrs. Harper reacts to Miss Strangeworth’s comment in "The Possibility of Evil"?

Shirley Jackson's "The Possibility of Evil" is a wonderful little short story that, like other works by Jackson, focuses on what happens when the idiosyncrasies of small town life go terribly wrong. Miss Strangeworth is a local gossip, but rather than share stories with others, she creates falsehoods and mails anonymous letters to those they concern, thus setting many in the town on edge and causing them to second-guess both themselves and those around them.

When Miss Strangeworth goes to the grocery store, she runs into Mrs. Harper, who seems somewhat nervous. Jackson writes:

Miss Strangeworth moved slightly to make room for Mrs. Harper at the counter.

“Morning, Adela,” Mrs. Harper said, and Miss Strangeworth said, “Good morning, Martha.” . . .

“Ran out of sugar for my cake frosting,” Mrs. Harper explained. Her hand shook slightly as she opened her pocketbook.

At this moment in the story, there is no explanation for Mrs. Harper's reaction, and so it's difficult to infer what is happening. However, toward the latter part of the story, Jackson allows the reader to witness Miss. Strangeworth's writing process. In doing so, she also presents more information regarding the potential reason for Mrs. Harper's nervousness:

After thinking for a minute, she decided that she would like to write another letter, perhaps to go to Mrs. Harper, to follow up the ones she had already mailed. She selected a green sheet this time and wrote quickly: Have you found out yet what they were all laughing about after you left the bridge club on Thursday? Or is the wife really the last one to know?

Clearly, Mrs. Harper has been a target of Miss. Strangeworth for a long time. The letter she is writing in this instance is a "follow up" to many that she has already written to Mrs. Harper. The subject matter of the letter itself makes it clear why Mrs. Harper reacted somewhat curtly and nervously: Mrs. Harper has been receiving anonymous letters suggesting that not only is her husband cheating on her, but everybody in town knows it and talks about it behind her back.

In case there were any doubt as to Miss. Strangeworth's horrid nature, Jackson goes on to state that "Miss Strangeworth never concerned herself with facts; her letters all dealt with the more negotiable stuff of suspicion." Clearly, she is terrible. Fortunately, by the end of the story, she slips up and is discovered. Unfortunately, though, the reader is never granted access to the fallout, beyond the suggestion that her roses have been destroyed.