So virtue is a purposive disposition, lying in a mean that is relative to us and determined by a rational principle, by that which a prudent man would use to determine it.

Virtue is subjective. Each person, or group of people, dictate what virtue is based on their values and opinions. If something is considered a virtue, it is because the person (people) in question can see and feel that it is right based on their social preferences and beliefs. If something is not considered a virtue, it is because it goes against the grain of their values and beliefs. So, virtue is nurture, not nature.