This is a lesson about sin. It defines sin as an offense against God and outlines various types of sins, including original sin and actual sin. The lesson examines the severity of sins, differentiating between mortal sins, which sever the relationship with God, and venial sins, which weaken it. It also addresses internal sins, sins of thought and desire, and the seven capital vices, which are considered the root of other sins. The lesson concludes with a discussion of temptation, occasions of sin, and cooperation in evil, emphasizing the importance of avoiding these situations.