Disciplined use of Reason is the good habit of having the diligence to take the time to look at all the facts/information available, analyze the data and reach logical and practical conclusions about the best course of action to take. The man who harnesses Reason build a discipline of engaging his intellect as a guiding principle in his life, refusing to be ruled simply by impulse and emotion. 

The vice opposing the Disciplined use of Reason is to be Irrational, a bad habit by which a man fails to use reason and is impulsive, is flawed and illogical in his conclusions, or over-relies on his emotions/passions to guide his decision making. The Irrational man often causes himself and others great suffering and confusion because his thoughts and actions defy common sense and predictability and lead to bad outcomes.