Mathilde lied about losing a valuable necklace she had borrowed and pretended to live a life she could not afford in order to keep up appearances. She lived unhappily with the consequences of her deception. The author argues people should be honest even when telling unpleasant truths, as honesty builds trust and allows one to be at peace with oneself and others. While lies may seem unimportant initially, being dishonest distorts one's perspective and causes greater problems than the truth.