The theme of "A Piece of String" has to do with the meaness, cruelty, and injustice of humanity. Maupassant often wrote stories about human selfishness, wickedness, envy, spite, greed, and other bad qualities. Here are a few pertinent quotations:
"Everyone is perfidious, a liar and a phony. Everyone wears a false face."---Guy de Maupassant
Maupassant persuades us to accept his illusion that cunning, ferocity, greed, and coarseness are more common among men than we hope they are.---Wallace Stegner
The thing that most tormented De Maupassant, to which he returns many times, is the painful state of loneliness, spiritual loneliness, of man, of that bar which stands between man and his fellows; a bar which, as he says, is the more painfully felt, the nearer the bodily connection.---Leo Tolstoy
Maupassant was deeply influenced by the pessimistic German philosopher Artur Schopenhauer, who had an even lower opinion of human nature. In "A Piece of String" an entire village makes a simple, humble man's life miserable by accusing him of theft. There is nothinig he can do to convince them that he was only picking up a piece of striing. His accusers don't want to believe him because they enjoy venting their spite on a defenseless man.
"A Piece of String" might be compared with Shirley Jackson's story "The Lottery" and with Maupassant's famous story "Boule de Suif."