“(The Buddha said) in the King of Samadhi: “Use your intellect to apply to all else your understanding of the self-concept – all things have an utterly pure nature just like the sky. By a single (insight), all things are known – by one (vision), all things are seen.” That is, if one knows well the real condition of the “I” which anchors the concept of self which thinks “I”, and one applies the reasoning about it to internal things such as eye and nose and external things such as vases, etc., one should come to understand them in just the same way. Then, knowing the nature and seeing the reality of one thing, one can be able to know and see the natures of all other things.”
From The Medium Treatise on the Stages of the Path to Enlightenment by Je Tsong Khapa, as published (insight section only) in The Life and Teachings of Tsong Khapa edited by Robert A.F. Thurman. Library of Tibetan Works and Archives.