
En Purgatorio IV encontramos por primera vez un tema que va a ser importante durante el resto de la Comedia:
Ed elli a me: "Questa montagna é tale,
che sempre al cominciar di sotto é grave;
e quant'uom piú va su, e men fa male.
Peró, quand'ella ti parrá soave
tanto, che su andar ti fia leggero
com'a seconda giú andar per nave,
allor sarai al fin d'esto sentero..." (Pur. 4, 88-92)
Y él (Virgilio) me dijo: "Esta montaña es tal
que siempre al comenzar, abajo, nos pesa más;
en cuanto vamos subiendo sentimos alivio.
Mas cuando ella te parezca tan fácil
como nave que va con la corriente,
entonces estarás al final de este sendero..."
Sobre esto comenta Jeffrey Schnapp en "Introduction to Purgatorio" en The Cambridge Companion to Dante:
Medieval Science, following Aristotle, viewed the sublunar world as fundamentally different from the heavens. Whereas the latter were perfect, regular, and perpetual, the former was characterized by its imperfection, irregularity, and temporal flux. This distinction helped scientists to explain why, for instance, the sun follows precisely the same course year after year, whereas cloud-formations do not. Belonging as they do to the lower atmosphere, clouds appear and disappear; their formations are provisional and sporadic. The same sort of mutability extends to animal and plant life. Mount Purgatory functions as a kind of ladder between these ontologically distinct domains. The Ante-Purgatory lies fully within the realm of generation and corruption and is subject to fluctuating weather and to natural events such as earthquakes. The rest of the mountain, from the gateway to the summit, is free from all forms of natural mutability (see 21, 43-72).
El mundo sublunar es fundamentalmente diferente al de los cielos...
La montaña del purgatorio funciona como una escalera entre estos dos ámbitos ontologicamente distintos...
El antepurgatorio yace dentro del ámbito de generación y corrupción y está sujeto a las fluctuaciones del clima y a accidentes naturales como terremotos...
El resto de la montaña, desde la entrada hasta la cima, está libre de todo tipo de mutabilidad natural.