大毘盧遮那成佛經疏 15.20

15.20

次答云何昇空界者，言行者乘悉地之力，遊步空界自在無礙，猶如於幻法中得無畏者。無所畏者，謂於幻法方軌法則悉能了知無疑難故，所欲成也。如彼幻師化作種種境界，能隨人所欲而為示現，然彼幻師實無動搖亦無有變異，但以真言藥物因緣惑眾人心故耳。今此悉地亦爾，不捨此身悉成悉地。真言幻猶，如帝釋網，而步虛空界遊諸剎土種種自在，然亦未曾動搖，但是真言幻所作故得如是耳。此幻師真言名帝釋網，猶彼網於一切事顯示相現有種種相，而實無有一一實事，故名帝釋網。此即是彼幻人世間真言。借以為喻，喻出世真言之幻也（又云網是不斷絕義，若網人則令人蒙昧不能自出。此真言網諸人心，亦彼蒙昧妄有所見，故名網也。）

Next, answering "how does one ascend to the realm of space?": it means that the practitioner, riding on the power of siddhi, roams through the realm of space freely and without obstruction, just like one who has attained fearlessness in illusory arts. "Having no fear" means that because one completely understands the methods, rules, and procedures in illusory arts without doubt or difficulty, whatever is desired is accomplished. Like an illusionist who conjures up various realms and can display them according to what people desire; yet that illusionist in reality has neither moved nor undergone variation, but merely deludes the minds of the multitude through the conditions of mantras and medicinal substances. Now this siddhi is also like this: without abandoning this body, one completely accomplishes siddhi. The illusion of mantras is like Indra's net; one treads the realm of space, roams through various lands, with various kinds of sovereignty, and yet has never moved; it is only because it is produced by the illusion of mantras that one attains this. This illusionist's mantra is called "Indra's net"; just as that net manifests appearances regarding all things, appearing to have various forms, yet there is not a single real thing—hence it is called Indra's net. This refers to the worldly mantras of that illusionist. It is borrowed as a metaphor to illustrate the illusion of supramundane mantras. (It is also said that "net" has the meaning of non-interruption; if a net catches people, it makes them bewildered so that they cannot free themselves. This mantra nets people's minds, and they too are bewildered and falsely perceive things; hence it is called a "net".)