首楞嚴義疏注經 5.304

5.304

初觀白相，經三七日後見息氣猶如煙相。此觀成時，身心內發，若身若器一時空淨，內外映徹，猶如琉璃。此則因觀生滅息相，觀心融明，將發空慧，遂洞身界。猶在方便，未能忘緣，故見其煙變成白相。

At first contemplating the mark of whiteness, after three periods of seven days he saw the breath like the mark of smoke. When this contemplation succeeded, body and mind opened up from within; both the body and the receptacle world simultaneously became empty and pure, mutually reflecting inside and out, just like vaidūrya. This was because, through contemplating the mark of the arising and ceasing of the breath, the contemplating mind blended and brightened; being about to generate empty wisdom, it accordingly penetrated body and realm. Still being in the stage of expedient means, he was not yet able to forget objective conditions, hence he saw that smoke turn into a mark of whiteness.