The key to understanding the Mahayana and Vajrayana views lies in Nagarjuna’s reasonings. This is because the reason the aggregates and suffering can be described as being pure by nature is that they are empty by nature—they are unborn. They never actually come into existence. Something that never really comes into existence cannot possibly be impure, for what is there to be impure in the first place? It is like getting covered with filth in a dream—no matter how dirty you might seem to be, since not a single particle of the filth is real, in fact there is no impurity at all. Since there is no impurity, there cannot actually be any purity either, just as when you take a bath in the dream after having gotten so filthy, your cleanliness after the bath is just as lacking in reality as the dirtiness that preceded it. Therefore, the true nature of the dream transcends both purity and impurity, and this is given the name “original purity.” We have to understand that what original purity refers to is the freedom from all fabrications, the emptiness in which we can gain certainty by using Nagarjuna’s reasonings.
From The Sun of Wisdom: Teachings on the Noble Nagarjuna’s Fundamental Wisdom of the Middle Way by Khenpo Tsultrim Gyamtso, page 80