These are notes from a devotional Elder Bednar gave at the MTC in Nov 2010.
Summary
Doctrine is truth, salvational truth, revealed from our Heavenly Father about our eternal progress. A principle is a guideline based on doctrine. And it is specifically intended so that we can righteously exercise our moral agency. Doctrine answers "why?", principle answers "what?", applications answer the question of "how?".
Doctrine
A doctrine almost always answers the question why?
A gospel doctrine is a truth. It is a truth revealed by Heavenly Father to His children here upon the earth. And that revealed truth pertains to our eternal progression. Doctrine is simple, doctrine is foundational, and doctrine is fundamental.
Doctrines are found in the scriptures. They are found in the teachings of the living apostles and prophets. You’ll hear it in General Conference. You find doctrines in the authorized proclamations and declarations that come from the First Presidency and the Twelve. And only the First Presidency and the President of the Church have the authority to definitively determine and declare the doctrines of the gospel of Jesus Christ.
Principle
A principle almost always answers the question what?
A principle is a guideline based on doctrine for the righteous exercise of moral agency.
Application
Applications answer the question of how?
Applications are the actions and things that we do. We focus on applications because we like to think we’re in control. "I’m going to manage this; I’m going to make it happen." ... there is no lasting spiritual impact from simply manipulating applications.
Conclusion
So how do you improve behavior? In a yourself, an investigator, or a child?
The answer is always the doctrine. Not in gimmicky applications, but in the simple pure doctrine and principles of the doctrine of Jesus Christ. A statement from President Packer, “true doctrine understood changes attitudes and behavior.”
Understanding, in the scriptures, is linked to the heart. For example, in Mosiah 12:27, note the relationship between heart and understanding. This is Abinadi teaching, “ye have not applied your hearts to understanding; therefore, ye have not been wise.” Wouldn’t you think that you would apply your mind to understanding? Certainly, but applying your mind to understanding is not enough. It’s a beginning, but it’s not enough. How does revelation come? Thoughts to the mind and feelings to the heart. Apply your heart to understanding. What is understanding? It’s the revealed result. When what we know in our head moves to our heart by the power of revelation, that’s the beginning of understanding. Understanding is what comes by the power of the Holy Ghost not because of the clarity of your explanation. You have to work to explain as clearly as you can, but the best explanation you ever give will never produce understanding in the mind of an investigator. Only the Holy Ghost can do that, and it will be in the heart and not in the mind.
Focusing on actions and behaviors will never change action and behavior in you, in me or in anybody else. The answer is the doctrine. The answer is always the doctrine.
Source:
Elder Bednar, The Answer is the Doctrine