Everywhere Aristotle utilizes the phases of qualities passing into and out of their opposite states as an anchor to the progress of his thinking. Generation and Corruption studied as contraries is fitting to this basic principle or method and it fits his contention that the Real is always at least two in number.
Coming to be necessarily implies the pre-existence of something which potentially 'is', but actually 'is not'; and this something is spoken of both as 'being' and as 'not-being'.
Coming to be occurs in the region about the centre. (of the Universe, i.e., Earth) The originative sources of generation are first matter and form. A third originative source must also be present for while it is characteristic of matter to suffer action, i.e. to be moved: to move, i.e. to act, belongs to a different 'power'. Things assume forms which are their essential nature.
The third originative source is the alternating approach and retreat of the Sun, which corresponds with generation and corruption. "Coming-to-be and passing-away will, as we have said, always be continuous, and will never fail owing to the cause we stated. And this continuity has a sufficient reason on our theory. For in all things, as we affirm, Nature always strives after 'the better'. Now 'being'... is better than 'not-being': but not all things can possess 'being', since they are too far removed from the 'originative source. 'God therefore adopted the remaining alternative, and fulfilled the perfection of the universe by making coming-to-be uninterrupted: for the greatest possible coherence would thus be secured to existence, because that 'coming-to-be should itself come-to-be perpetually' is the closest approximation to eternal being."
The cause of this perpetuity of generation is circular motion: for that is the only motion which is continuous. "... if there is to be movement... there must be something which initiates it; if there is to be movement always, there must always be something which initiates it; if the movement is to be continuous, what initiates it must be single, unmoved, ungenerated, and incapable of 'alteration'; and if the circular movements are more than one, their initiating causes must all of them, in spite of their plurality, be in some way subordinated to a single 'originative source'."
"...a thing is eternal if its 'being' is necessary: and if it is eternal, its 'being' is necessary. And if, therefore, the 'coming-to-be' of a thing is necessary, its 'coming-to-be' is eternal; and if eternal necessary.
"It follows that the coming-to-be of anything, if it is absolutely necessary, must be cyclical---i.e. must return upon itself....
"The result we have reached is logically concordant with the eternity of circular motion, i.e. the eternity of the revolution of the heavens."
I seize on a couple of statements here in particular, for instance, coming to be is an effort by grace to mirror the divine's eternal being by itself being a coming to be perpetually; he says a gift of God. And the idea put forth that generation being circular exhibits the characteristic of returning on itself echos the notion that the essential nature of consciousness shares the same trait.
So, the center, Earth, is where generation and corruption take place and the ultimate cause is bound up in, is made perceptible, by the circular movement of the fixed stars in the outer most sphere, that is, heaven. The abode of the imperishable and perfect affects all the coming to be and passing away we creatures suffer in the lowest regions, the Earth. The Sun is made to follow its alternating approach and retreat by the higher revolutions of the outermost sphere of fixed stars. It in turn imparts the necessity of generation and corruption to what is beneath its orbit. Aristotle further assigns decreasing powers to the elements both primary and simple being Earth, Air, Fire, Water which in their turn come and go in their own orbits alternatively creating and destroying what is within their purviews.
Next, De Anima
