Mentoring is the fastest way for one to learn or relearn first principles. Explaining an idea to someone requires that you speak to their level of understanding. Thus, explaining a complex idea to someone with understanding of only fundamental truths (novices), requires that you only speak in first principles. If you cannot, you will provide a bad explanation. When you eventually are able to provide a good explanation, you will know you have grasped the fundamental truths of the idea.