Buddhist meditation practice
Buddha contemplation (Chinese: "guānfo" 觀佛), is a central Buddhist meditation practice in East Asian Buddhism, especially popular in Pure Land Buddhism, but also found in other traditions such as East Asian Yogācāra, Tiantai and Huayan. This practice involves the visualization and contemplation of a mental image of a Buddha and the attributes of their Pure Land, aiming to develop faith, devotion, and a deep connection to the Buddha's spiritual qualities. As such, Buddha contemplation is a Mahayana type of "buddha mindfulness" (buddhānusmṛti) meditation which focuses on imagination or visualization. The most popular Buddha used in this practice is Amitābha, but other figures are also used, like Guanyin, Maitreya, Cundi, and Samantabhadra.
The practice of Buddha contemplation is taught in various Mahayana sutras called Contemplation Sutras (, "Guān jīng", sometimes also translated as Visualization Sutras)"," which teaches contemplative practices based on fantastic visual images of Buddhas, bodhisattvas and their buddhafields. These works mostly survive in Chinese translations dating from about the sixth century CE. In Pure Land Buddhism, one of the most important sutras is the "Amitāyus Contemplation Sūtra". 
Overview.
Buddha contemplation is a kind of buddhānusmṛti (Buddha recollection, remembering the Buddha), a classic Buddhist meditation taught in numerous Early Buddhist sources which focused on contemplating the qualities of the Buddha, including the physical qualities of his body.
One of the earliest Mahayana sutras which describes something like the practice of Buddha contemplation (or at least visions of Buddhas) is the "Pratyutpanna Samādhi Sūtra," whose full title is "Pratyutpannabuddha Saṃmukhāvasthita Samādhi Sūtra" ("Sūtra on the Samādhi for Encountering Face-to-Face the Buddhas of the Present"). In this sutra, the Buddha states that one may go to a secluded spot, sit down and concentrate one's thoughts on the Buddha Amitayus, then:If they concentrate their thoughts with undistracted minds on the Tathagata Amitayus for seven days and nights, then, when a full seven days and nights have elapsed, they see the Lord and Tathagata Amitayus. Should they not see the Lord during the daytime, then the Lord and Tathagata Amitayus will show his face to them in a dream while they are sleeping. In Pure Land Buddhism, "guānfo" is primarily associated with the meditation on Amitābha Buddha and his Western Pure Land, Sukhāvatī, a realm of bliss and enlightenment. Practitioners visualize Amitābha surrounded by serene landscapes and the Pure Land's inhabitants, such as bodhisattvas and celestial beings. They may also practice any of the sixteen contemplations or visualizations taught in the "Contemplation Sūtra of Amitāyus", which includes visualization of the sun, of the ponds and trees of the Pure land and of the bodhisattvas Guanyin and Dàshìzhì.
In the "Amitāyus Contemplation Sutra," Queen Vaidehi asks the Buddha to teach her how to be reborn in the Pure Land of Sukhavati, and the Buddha states that she must “fix your thoughts and clearly contemplate that [buddha] land" and that through this practice, and "the power of the Buddha", "you will be able to see that Pure Land as clearly as if looking at your own image in a bright mirror. Seeing the utmost beauty and bliss of that land, you will rejoice and immediately attain insight into the non-arising of all dharmas." In explaining how this practice is effective, the Contemplation sutra states:Because each Buddha Tathagata, as the body of the dharma-realm, pervades the mind of all sentient beings. Therefore, when you perceive a Buddha in your mind, it is your mind that possesses the thirty-two  prominent features and the eighty secondary attributes; your mind  becomes a Buddha; your mind is a Buddha; and the wisdom of the Buddhas — true, universal, and ocean-like — arises from this mind. Therefore, you should single-mindedly fix your thoughts and clearly perceive the Buddha... The "Contemplation sutra" and its teaching on Buddha contemplation was very influential on East Asian Buddhism, especially the Pure Land school and there are over forty commentaries composed on it from the Sui to the Song dynasty alone. In Pure Land Buddhism, the visualization meditations are often accompanied by recitation of the Buddha’s name ("nianfo" in Chinese). It is believed that through such practices, the Buddha's power will purify the minds of practitioners, and that they will attain birth in the pure land.
The practice of Buddha Contemplation is taught in several other texts which (together with the "Amitāyus Contemplation Sutra") are known as "Contemplation Sutras" (觀經, "Guān jīng"). A main feature of these Contemplation Sutras is their teaching of contemplative practices using vivid visual imagery associated with Buddhas and other Buddhist deities. However, according to David Quinter, the contemplations taught in these sutras:embrace more than visual phenomena, including auditory and didactic elements. Some passages do appear to urge the kind of precise visual replication of phenomena in the mind’s eye that is typically understood by “eidetic contemplation,” and which is integral to many uses of “visualization” in English. But other passages...point more toward any resulting vision as confirmation of the success of one’s practice, and these visions do not always mirror the phenomena described. There is no consensus on a Sanskrit basis for the term "guan" (觀, which can mean contemplation or visualization). It could have referred to buddhānusmṛti (buddha recollection) or to dhyāna (meditation). According to some modern scholars, various Central Asian and Chinese Buddhist cave sites also include artistic works which are related to these contemplative images. In some of the Contemplation Sutras, like the "Samadhi Sea sutra", the use of Buddha images or statues for devotion and meditation is specifically taught. 
The Contemplation Sutras also often discuss and teach the "recollection" or "oral recitation" (Chinese: nian 念) of the Buddhas and bodhisattvas, indicating that the practice of "contemplation" (guan) was more than just simple visualization. As Luis Goméz writes in his study of the Pure Land sutras, the term "guan" is not just visualization but:[a] dedicated and constant repetition of verbal imagery— a sort of narrative rehearsal (anusmṛti?). This practice overlaps with certain forms of meditation but also overlaps with other rituals of remembrance and devotion. In practice, the Meditation and the Shorter Sutras often provide the content for chanting and recitation, rather than for silent meditations. The ritual and devotional context in which one finds these sutras fits somewhere between reciting or rehearsing a narrative, chanting a litany, imagining a narrative setting, and meditating. Similarly, according to scholars like Nobuyoshi Yamabe and Cuong Mai, the practice of contemplation (guan) was closely connected with the practice of recitation or chanting (either of a ritual formula, a sutra or a Buddha's name) in Central Asian Buddhism and in early Chinese Buddhism. 
Furthermore, the various Contemplation sutras also discuss how the practice of contemplation (guan) is effective at repenting for, purifying and extinguishing the effect of bad actions (i.e. evil karma) done in past lives. 
Buddha contemplation is similar to and historically precedes the Vajrayana practice of Deity yoga. Some scholars like Robert Sharf see Buddha contemplation sources as being precursors to deity yoga. Sharf writes that the "Contemplation Sutras" contain meditations that include recitation, the use of icons, visualization , "and other elements often associated with Tantra". However, unlike the tantric deity yoga, Buddha contemplation does not require esoteric initiation (abhiseka), or make use of esoteric mudras or mandalas.
Sources.
The Contemplation Sutras.
There are various Mahayana sutras associated with the term "guan", though generally six major texts as seen as the central "Contemplation sutras" (, "Guān jīng") as listed by Alexander Coburn Soper (1959).
While the Chinese Buddhist canonical tradition claims these sutras are translations from Indian texts, no Indic originals have been found. Scholars disagree on their origin, positing Indian, Central Asian, or Chinese origins for specific sutras. While many scholars argue they were composed in China, even then, they accept the influence of Indic and Central Asian meditation traditions. 
The main Contemplation Sutras are: 
Two other texts might also qualify as "Contemplation Sutras" or as being from a similar religious milieu. The first is the "Mañjuśrī Parinirvāṇa Sutra" (Wenshushili banniepan jing 文殊師利般涅槃經; T 463). 
Another possible Contemplation type sutra is a part of the "Avalokitasvara Invitation Sutra" (Qing Guanyin jing 請觀音經; T 1043), particularly the chapter on Upasena. According to Greene, this section may be a part of the lost "Avalokiteśvara Contemplation Sutra" (Guanshiyin guan jing 觀 世音觀經) which appears as extant in Sengyou’s 517 catalogue but is listed as lost in later sutra catalogues. 
Other related sources.
Nobuyoshi Yamabe notes that the following texts also have a similarity to the visualization sutras, some of these are part of the so called "Meditation Sutra" category (Ch: "chanjing"):
Yamabe also cites various practice manuals found at Dunhuang, such as the "Sūtra on the Major and Minor Bodily Marks" (Xianghao jing 相好經), which is based on the Samādhi Sea Sutra. 
References.
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