Commentary Par I 112-117

Reverting to nautical metaphor (see [Par I 67-69]) for the life-journeys of all created things, whether capable of willing or not, the poet equates the purposes of inanimate and one kind of animate life with voyages toward various ports, the ends for which He has ordained these of his creations.  This impulse is exemplified in three kinds of things: a light element (fire) with its inherent 'desire' to rise to its own sphere (see C.Par.I.61-63); creatures possessed of an animal (but not a rational) soul; and a heavy element (earth) with its obedience to the law of gravity, expressed as a 'desire' to become compacted (the opposite impulse from that of fire).