Theories accurately describe why specific results have been achieved in the past; and why, given the same circumstances, the same result will occur in the future.
We tend to underestimate the importance of theory in most of the activities in our lives. The overlooked importance of theory has a particular importance in economics. Economists frequently confuse correlation with causation. By not testing observations with sound theory they frequently recommend actions detrimental to their desired results.
Principles consist of assumptions that provide the basis for sound economic reasoning. In terms of logic they amount to truths. Starting with these principles and using valid argument we can arrive at only sound propositions.