— 青原惟信禅师 (意译)
As above, it was proposed by Chán Master Qīngyuán Wéixìn in his paraphrased saying, there are three key stages to be experienced when practising meditation. The following is the actual record of his words in ‘Record Of Pointing At The Moon: Scroll 28: The 6th Patriarch’s 14th Generation’《指月录·卷二十八·六祖下第十四世》:
‘This old monk, thirty years ago, when yet to practise meditation,  saw mountains as mountains, and saw rivers as rivers. Up until later, when personally seeing to know, that there is an entrance point,  to see mountain not as mountains, and see rivers not as rivers. Yet now, having attained a resting point, accordingly as before,  saw mountains only as mountains, and saw waters only as waters. Great assembly, of these three kinds of views and understanding, are they the same or are they different? If there are people, be they monastics or lay practitioners, who can arrive at a conclusion, perhaps you personally do see this old monk.’ (老僧三十年前，未参禅时， 见山是山，见水是水。及至后来亲见知识，有个入处， 见山不是山，见水不是水。而今得个休歇处，依前  见山只是山，见水只是水。大众，这三般见解，是同是别？有人缁素得出，许汝亲见老僧。) With several ways to explain these stages, below is one.
 At the beginning of practice, there is seeing of mountains as mountains, and rivers as rivers. This is the ordinary or unenlightened mind, that only sees forms as they appear to be, as the forms they ‘are’ (色即是色). This is with fixed (mis)perceptions, such that seeing is habitually judgemental and superficial. There is seeing of all forms as constant, thus permanent (常), with fixed natures, thus with self (我), with each being unconnected from one another, thus independent. It is with much discrimination (分别) and thus limited, far from perceiving further. Such is the mind that despite seeing many forms, does not realise they are all but separately and falsely named (假名) for conventional communication. This is the mind that conditions suffering (苦).
 When having some initial awakening from practice, there is seeing of mountains not as mountains, and rivers not as rivers. This is the mind that has at least caught a glimpse of forms as emptiness (色即是空), not merely as what they appeared to be. This is with more open and fluid perception, such that seeing is more thorough and deeper. There is seeing of all forms as constantly changing, thus impermanent (无常), without fixed natures, thus without self (无我). Seen within forms of the biggest mountains is nothing but emptiness, just as forms of rivers are too. This state might seem to be without discrimination and thus unbound, but if clung to, is a form of discrimination and still limited, yet to perceive completely. Such is the mind that having seen emptiness, might become attached to it. This is the mind that conditions self-liberation, but yet to be fully enlightened, to be able to guide all willingly and skilfully to liberation.
 When having thorough awakening from practice, there is still seeing of mountains as mountains, and rivers as rivers. However, there are big differences from the first stage. Seen within elements of mountains are elements of rivers and more, just as within elements of rivers are elements of mountains and more, with all interconnected and interdependent. This is the mind that has seen emptiness as all forms (空即是色) too, striking a balance on the (iii) Middle Path (中道). This is the most free perception, such that seeing is complete and penetrating. There is seeing of (i) forms as emptiness and (ii) emptiness as forms at the same time. (i), (ii) and (iii) are together called the Three Truths (三谛), with which, seeing phenomena of mind and matter as they truly are, enables no particular attachment to forms, emptiness or even the Middle Path. This is the mind that conditions complete liberation, to attain great ease that functions with great compassion and wisdom to guide all to Buddhahood. That said, it is with birth in Amitābha Buddha’s (阿弥陀佛) Pure Land (净土), that there can swiftest complete realisation of all Three Truths.
Verse On Mountains & Rivers
 See mountains and rivers as the Falsely Named,
 see mountains and rivers as Emptiness,
 and see mountains and rivers as the Middle Path,
to see the Three Truths completely.