“Truly we often see things which provide false matter for misdoubting, just because their real causes deep within them hide”
. . . . .
An unidentified soul. The figure explains that the tremors happen whenever a newly purified soul is released to Heaven—when the soul is free to choose Heaven for itself. The soul confirms the teaching that Virgil introduced earlier—those souls, when fully purified, are not so much granted release from Purgatory as they are able to freely choose Heaven for themselves for the first time.