What is the principle?
10% of life is made up of what happens to us. 90% of life is decided by how we react.We have no control over this 10%, but we can determine the other 90% by control our reaction. Don't let people fool us.
Very buddhistic teaching indeed, to keep it simple: I guess Stephen Covy is trying to say, be equanimous all the time! We would be steady in going through all the life challenges.