A commonsense theory of nonmonotonic reasoning is presented which models our intuitive ability to reason about defaults. The concepts of this theory do not involve mathematical fixed points, but instead are explicitly defined in a monotonic modal quantificational logic which captures the modal notion of logical truth. The axioms and inference rules of this modal logic are described herein along with some basic theorems about nonmonotonic reasoning. An application to solving the frame problem in robot plan formation is presented.