A teacher affects eternity; he can never tell where his influence stops.

The line emphasizes the long, often untraceable afterlife of teaching. A teacher’s words, habits, and example propagate through students into families, workplaces, and future generations, creating effects that outlast any single classroom moment. The claim that a teacher “affects eternity” is not literal prediction but a moral and philosophical reminder: education is a form of stewardship whose consequences cannot be fully measured or controlled. It also implies humility—because influence is diffuse and cumulative, teachers cannot know which lesson will matter most, or how it will be transformed as it passes from person to person.