In simple words, pure perception is when you see everything as manifestation of Buddha-nature. Even things we usually consider bad or imperfect you see as a part of Great Perfection at a deeper level.
...pure perception is the main view and practice on the Vajrayana path. There is no room whatsoever for even a glimmer of an impure perception.
But what is ‘pure perception’? Ultimately, according to the Vajrayana, ... the main point of pure perception is to go beyond dualistic perception altogether and realize the union of emptiness and appearance.
To put it simply, pure perception is the highest form of mind training—dag nang byang in Tibetan. Dag means ‘pure;’ nang means ‘perception,’ and byang means ‘train’ or ‘get used to.’
So, how does pure perception work? As a Vajrayana student, if you look at [your guru] and think he’s overweight, that is an impure perception. To try to correct your impure perception you might then try visualizing him with the body of Tom Cruise, but that is still not pure perception. One of the Vajrayana’s infinite number of skilful methods that are used to deconstruct and dismantle impure perception, is to visualize [your teacher] with a horse’s head, a thousand arms and four legs. But even this technique must ultimately be transcended in order to fully realize pure perception.
Basically, while the student’s perception remains impure, the guru they see will be a projection based on their own impure projection, and so it can only ever be imperfect. The only way we can change our impure perception and see the guru as an enlightened being is by training our minds, using the visualization practises provided by the Vajrayana path.
No Vajrayana teaching or qualified Vajrayana teacher would ever expect a student’s perceptions to be completely pure from the moment they step onto the Vajrayana path. This is why the techniques we apply are called ‘training’—and even the English word ‘training’ implies that mistakes are inevitable. But there’s a very simple way of checking your progress with this practice. In the Vajrayana, you are supposed to see not only the guru but yourself as a deity. So if, having just been taught that you are a deity, you skip lunch and feel hungry, it means your training is not complete. You will only be perfectly trained in pure perception once you have finally actualized the union of appearance and emptiness.
So if a student ... were to see [his guru] floundering in the middle of a lake and based on their impure perception, project onto him the idea that he seems to be drowning, it would probably not be a good idea for that student to think, “Rinpoche is an enlightened being and should be able to walk on water.” A much better thought would be, “This is my impure perception! Rinpoche is manifesting as a drowning man so that I can accumulate the merit of rescuing him.”
As your practise improves, your perception of the guru will no longer be bound or limited by the causes, conditions and effects that once made you think he was drowning. This is the point in your spiritual development when you will truly see the outer guru as the Buddha and will also be able to see your own inner guru.
Until then, when your guru chairs a board meeting and it becomes obvious that he has no clue about an issue, as a prudent member of that board you shouldn’t hesitate to supply him with the information he needs. At the same time, as a Vajrayana student, you must skilfully remind yourself the guru only looks clueless to you because of your own impure perception, and that by appearing to need your assistance the guru is actually giving you the chance to accumulate merit.
We all have habits, and it’s habit that makes impure perception inevitable. The moment we step onto the Vajrayana path, we start breaking ‘samayas’—which are our commitment to maintaining pure perception. This is why the assumption that all Vajrayana practitioners will make mistakes is built into the Vajrayana path. A practitioner’s path is then to immediately confess, expose and fix any impure perceptions the moment they arise, and to continually aspire to make fewer and fewer mistakes.
This is what is meant by keeping the samaya vows. In fact, Vajrayana practice cannot be separated from keeping samaya. There is no such thing as: “Let’s keep samaya and then practice.”
In Buddhadharma, not just the Vajrayana, the only way any of us can keep [pure perception], is by fully realizing a perfect understanding of shunyata.
In Buddhism, the general idea is that we train our minds to actualize non-duality. Tantra offers us the most profound way of achieving that non-duality through the practice of pure perception; and in the Vajrayana we essentialize that practice by maintaining a pure perception of the guru.
Ultimately, as Vajrayana practitioners, we must apply pure perception to everyone and everything without exception, which means we must also apply it to Donald Trump and even Hitler. But we will only manage to achieve a pure perception of everyone and everything if we can first maintain a pure perception of our guru. If you try to retain the option of questioning, criticizing and analyzing—in other words if you retain some kind of selective impure perception as an insurance policy that allows you to question your very path—then how will you achieve the cessation of the dualistic mind? How will ‘one taste’ be actualized? How will you realize the union of samsara and nirvana?
On the more advanced level, pure perception is when you know that any observable reality is our interpretation and you can see in real time how your mind overlays an interpretation and simultaneously see inside that interpretation and outside of it. This is what's called "realize a perfect understanding of shunyata" and "actualize the union of appearance and emptiness".