The Nyingma School of Tibetan Buddhism: It's Fundamentals and History by Dudjom Rinpoche & Jikdrel Yeshe Dorje
Chapter 8 The Superiority of Atiyoga, the Great Perfection
Thirdly, [following Mahayoga and Anuyoga,] there is the explanation of the definitive order of Atiyoga, the Great Perfection, which is the climax of all vehicles and the culmination of all [paths] to be traversed. This is revealed in two parts: its superiority over the lower [vehicles] and its divisions.
As to the first: This king among vehicles holds the expanse [of reality], the originally pure mind-as-such whose natural expression is inner radiance, and the naturally present, unchanging, pristine cognition that spontaneously abides in oneself to be the ground of great perfection. in the estimation [of atiyoga], the vehicles of the eight lower sequences have intellectually contrived and altered that which is unchanging exclusively through their suddenly arisen ideas which never experience what is in fact so. They have applied an antidote to and abandoned that which is not to be renounced. They have referred to as flawed that in which there is nothing to be clarified, with an intellect which desires clarification. They have induced dissension with respect to that which is not to be obtained by their hopes and doubts that it is to be elsewhere obtained; and they have obscured the pristine cognition, which intrinsically abides, by their strenuous efforts, with respect to that which is effortlessly present. Hence, they have had no occasion to make contact with the reality of the fundamental nature.
In this way, the pious attendants and self centered buddhas among the lower vehicles, with reference to the selflessness which they realize, hold consciousness and atomic matter to be the ultimate realities; and the proponents of consciousness (Vijnanavada) who hold consciousness, self-cognisant and self-reliant in nature, to be the absolute characteristic of ultimate reality, do not transcend [the view of] mind and mental events harbored by mundane beings. The Madhyamaka adhere to a truth of cessation scrutinized by four great axioms and the like, concerning the absence of creation, absence of cessation, absence of being and absence of non-being, which are appraised according to the two truths. And they adhere to an emptiness which is, for example, like the sky, representative of freedom from extremes and freedom from conceptual elaboration and so forth…
(and further down...)
In short, all these sequences [of the vehicle], from anuyoga downwards, are exclusively spiritual and philosophical systems contacted through the intellect. All of them, on the surface of the intellect, produce such thoughts as, "this is non-existent, this (is?) empty and this is true." Apart from this and their convictions and their boasting through ideas and scrutiny that reality lies within the subject - object dichotomy, they do not perceive the abiding nature of the natural state, just as it is.
Accordingly, it is said in the 'Sutra of the Nucleus of the Tathagatha':
The king assembled many blind men and, showing them an elephant, commanded, "Describe its particular characteristics." Those among them who felt the elephant's nose said it resembled an iron hook. Those who felt the eyes said that they resembled bowls. Those who felt the ears said they resembled winnowing baskets. Those who felt the back said it resembled a sedan chair, and those who felt the tail said it resembled a string. Indeed, though they did not describe the elephant as anything else, they were lacking in overall understanding. Similarly, though the nature of the Buddha is diversely described as emptiness, as illusory, as inner radiance and so forth, there is no overall understanding.
These paths have obscured the meaning of the Great Perfection, and if one develops their realizations while abiding in the path of the Great Perfection, it is explained to be a point of deviation (gol-sa). The All Accomplishing King says:
The six vehicles of definitive attainment are taught to be devotion points according to the Great Perfection. If one asks how this is the case, the sutra of the bodhisattva [vehicle] upheld the level of Universal Light. Through ideas and scrutiny concerning the two truths they hold reality to be empty as the sky. The supreme bliss of atiyoga, however, is the enlightened mind transcending ideas and scrutiny. That which transcends ideas and scrutiny is obscured by the sutras. Ideas and scrutiny, according to the Great Perfection, are explained to be the deviation in the sutra.
(and further down)
The Anuyoga upholds indivisibility. Having entered through the expanse and pristine cognition, the things which accordingly appear include the cause, which is the view of pure expanse and the result, which is viewed as the mandala of pristine cognition. The supreme bliss of Atiyoga, however is the enlightened mind transcending cause and result. That which transcends cause and result is obscured by Anuyoga. To behold a duality of cause and result, according to the Great perfection is explained to be the deviation in Anuyoga.
These [sequences of the vehicles] are created and fabricated by the mind, and yet [they hold that] the mind inclusive of the ideas they present in all eight aggregates [of consciousness] is a stain to be rejected. This natural Great Perfection, on the other hand, refers to mind-as-such transcending the mind, the uncompounded inner radiance of pristine cognition which is the natural presence of awareness, in which all the enlightened attributes of fundamental nature are spontaneously present. Apart from that, its essential point is that the abiding nature, characterized in the manner of the sky as unchanging, does not need to refer to causes and results that are either to be created or are in the process of creation, or to extraneous conditions and such elements, because it is naturally free from them. Thereby, [the Great Perfection] teaches that the nature of the Primordial Lord, Samantabhadra, is the buddhahood attained without contrivance, by realizing in one's own nature the naturally present pristine cognition, and that it does not otherwise emerge through extraneous conditions such as study, reflection and the accumulations of compounded provisions.