大毘盧遮那成佛經疏 14.47

14.47

凡行者持誦時，當觀字輪或為句輪。所謂句輪者，觀本尊心上有圓明，而布真言之字輪轉相接，令明了現前。持誦時觀此字猶如白乳，次第流注入行者口或注其頂，相續不絕遍滿其身乃至遍於支分。其圓明中字常明了，如常流水而無有盡。如是持誦疲極已，即但住於寂心，謂觀種子字也。其觀法如上已具說之。若得見種子字已，即從種子字中而見本尊也。如是成已，即能遍布字輪而成持明之體，方堪作諸事業也。如是阿字是不動義、是金剛體，凡欲令事堅固不動，或令若自若他道心不動等，皆用阿字加之。餘一切字亦隨事相應用之也。

In general, when the practitioner recites, they should visualize the wheel of syllables or the wheel of phrases. As for the wheel of phrases, one visualizes a round brightness upon the heart of the primary deity, and arranges the syllables of the mantra revolving in continuous connection, making them clearly present before oneself. While reciting, one visualizes these syllables like white milk, consecutively flowing and pouring into the practitioner's mouth or pouring upon their crown, uninterruptedly filling their body and even pervading their limbs. The syllables within the round brightness are always clear, like constantly flowing water without exhaustion. When one has recited like this and becomes exhausted, one simply abides in a tranquil mind, which means visualizing the bīja syllable. The method of this visualization has already been fully explained above. Once one attains sight of the bīja syllable, one sees the primary deity emerge from within the bīja syllable. When this is accomplished, one can fully distribute the wheel of syllables to form the body of a vidyādhara; only then is one fit to perform all ritual activities. In this way, the syllable A has the meaning of unmoving and is the vajra essence. Whenever one wishes to make an activity solid and unmoving, or make the resolve for the Path unmoving in oneself or others, one applies the syllable A to it. All the remaining syllables are also applied according to corresponding matters.