The Views of the Tenets, Vancouver 1991 གྲུབ་མཐའི་ལྟ་བ།

The essential nature of buddhahood pervades all beings without exception
Buddha nature is the fundamental nature of the mind whose essence is empty and who's makeup is clarity. This nature of mind cannot be said to be a thing, it is also not a non-thing, it is not something which one could describes as existing, nor can one describe as non-existing. Since it's pure nature is beyond any thought of the conceptual elaborations or intellectual ideas which one could use in describing it.
Rinpoche mentions a list of texts which describe the views of the different schools.

English translation by Jim Scott.