Principle: It is healthy for children to engage in an activity that promotes their intellectual development only if engaging in that activity does not detract from their social development  Application: Although Megan' s frequent reading stimulates her intellectually, it reduces the amount of time she spends interacting with other people
Ttherefore, it is not healthy for her to read as much as she does
It takes a necessary condition for an activity's being healthy as a sufficient condition for its being so.