Although Thomas Aquinas, following St. Augustine's lead, held that all lying is bad, what he called a “sin [peccatum],” he nevertheless believed that a harmless lie to save a life is only a venial sin, not a mortal sin. Still, some interpreters of Thomas would like to eliminate even this stricture, and to do so, they sometimes attempt to redefine the event of which Thomas is speaking. In so doing, they tend to undermine our identification of species of sin. They cast doubt on the validity of moral taxonomy....

Although Thomas Aquinas, following St. Augustine's lead, held that all lying is bad, what he called a “sin [peccatum],” he nevertheless believed that a harmless lie to save a life is only a venial sin, not a mortal sin. Still, some interpreters of Thomas would like to eliminate even this stricture, and to do so, they sometimes attempt to redefine the event of which Thomas is speaking. In so doing, they tend to undermine our identification of species of sin. They cast doubt on the validity of moral taxonomy. This chapter recalls some features of Thomas's doctrine of venial sin and considers how it applies to the discussion of lying.