"In Dzogchen, while thoughts are active, rigpa permeates them all, so that even at the very moment when powerful thoughts like attachment and aversion are arising, there remains a pervasive quality of clear light rigpa. Dodrupchen says, "in Dzogchen, since the clear light's natural way of being is like the sun and its rays, inseparable, if you are able, through this, to bring out the radiance of genuine mind, you will be able to maintain the experience of clear light in meditation, without it fluctuating, or coming and going."
Longchen Rabjam speaks of self-arising wisdom, which is in fact rigpa: "Self-arising wisdom is rigpa that is empty, clear and free from all elaboration, like an immaculate sphere of crystal. Its very being is such that it never explores objects of the senses."
This "self-arising wisdom" is rigpa, which in essence is primordially pure. Longchenpa describes it as "empty and clear". To call it empty is to refer to its essence, primordially pure. To call it clear is to speak of its nature, spontaneously present. As such, it is "free from all elaboration", and free from the elaborations of adventitious phenomena. So it is like a flawless crystal sphere, and truly "its very being is such that it never explores objects of the senses". (p.180-5)
--from Dzogchen: The Heart Essence of the Great Perfection by His Holiness the Dalai Lama, translated by Thupten Jinpa and Richard Barron, Foreword by Sogyal Rinpoche, edited by Patrick Gaffney, published by Snow Lion Publications