Answer: concept of "civil society" is based on the assumtion that all the people are rational and, consequently, can give to themselves rationally organized social order. This is the basis of what we call "civil society" or "social contract". Social contract is a rational concept based on idea of rational human being. "State of nature" is not considered rational, i.e. it is not harmonious. "Ratio" is considered as a factor that can create and creates social harmony.
Explanation: both have pros and cons. As - much later - Freud said = social progress is fed by human instinct, i.e. it takes place in the detriment of human instinct. That is the price modern societies pay for "civil society". "State of nature" is much more instinctual and irrational and consequently not very harmonious.