Aristotle on Coming To Be and Deterioration

I've been working on my Greek lately.  Here is a very rough translation of the beginning of Aristotle's On Generation and Corruption.  

Aristotle: Concerning Coming to Be and Deterioration  A

[314a1]  Concerning coming to be and deterioration, it is first necessary to distinguish the causes and accounts of those things that come to be and deteriorate by nature as well as all similar things.  Further, it is necessary to study growth and alteration, what each of them is, and whether one must understand alteration and coming to be to be the same by nature or distinct, just as we distinguish them by name.  

[314a6].  Of the ancients, some asserted that so called coming to be in the unqualified sense was the same alteration; however, others said that alteration and coming to be were different. 

 [314a8]  For those who argue that the whole is some one thing (they generate everything from that one thing), of necessity say that generation is alteration and that whatever is generated in the authoritative sense is altered.  But those who reckon the matter of things to be more than one, such as Empedocles, Anaxagoras, and Leucippus, hold that they differ.  

[314a13]  However, Anaxagoras does not understand his own saying.  At any rate, he says that generation and alteration are the same as being altered; however, along with others, he sets out the elements as many.  

[314a16]  For Empedocles, the corporeals are four and all of them including the kinetics are six in number.  For Anaxagoras, Leucippus, and Democritus, they are infinite (the one posits the elements to be homeomeries, such as bone, flesh, marrow, and everything else such that the part is synonymous with all of it.  Democritus and Leucippus say they are composed of indivisible bodies.  These are infinite both in number and form, the compounds differing from one another according to their position and arrangement.  

[314a24]  For the followers of Anaxagoras appear to be saying the opposite of the followers of Empedocles.  He says that fire, water, air, and earth are the four elements and so are simple, rather than the flesh, bone, and the other homeomeries.  

[314a28]  The former call these the simple things and the elements; earth, fire, water, and air are compounded panspermia of the former.  

[314b1]  Those who furnish everything from out of one must say that generation and corruption are alteration, must always hold that the underlying thing is one and the same (change in this we call alteration).

Commentary

The central problem of ancient philosophy was the problem of change and coming to be.  For something to change it must become what it is not, or else there is no change, and it must remain what it is, or else it has been destroyed and replaced by something else.  Yet becoming what it is not and remaining what it is seem to be logically exclusive.  Likewise, for something to come to be, it would appear that coming to be is something that happens to something; yet, what can happen to something that has not yet come to be? 

Aristotle begins (314a6) by distinguishing the monists from the materialists.  The monists solve the problem of coming to be by arguing that there is only one thing, the whole, and that thing is eternal.  If that is so, then there is no coming to be in the unqualified sense.  Nothing can come to be if there is only one thing.  In that case, all coming to be is merely alteration.  

Aristotle seems to believe that the reverse is true for the materialists who recognize a number of elements out of which all things emerge.  They must argue that coming to be and alteration are fundamentally different.  It is not yet apparent here why Aristotle believes this.