Rambam's short chapters concern his most dangerous and heretical theories: he did not want too much attention paid to them by the wrong crowd.
That means: when we see a short chapter, pay very close attention!
Rambam now takes us down a very Classical path: in order to later articulate his concept of the "perfectability of Man", he must first define the concept of a "form", because without an "ideal Form" of some sort, you can't even define perfection!
He did this without being sentenced to death by his own community, which means he had to write this portion in a particularly esoteric manner. That he does so with surgical precision only makes his sixth chapter most delightful.
Aristotle's Position on Form & Matter
Aristotle believed that "things" and phenomenona were a union of form and matter.
Delicately Weaving Forms: From Men & Women to Male & Female and then an Abstract Complementary Union
Rambam starts with the primordial: ish and ishshah, which are nouns in Hebrew that denote man and woman, respectively.
He then says these words, which initially only applied to men and women, were later applied to male and female animals in the Bible. THIS IS HIS PROOF THAT GENDER IS A FORM: the concept of the Form(in this case, gender) was an abstraction!
The two Hebrew nouns ish and ishshah were originally employed to designate the "male and female" of human beings, but were afterwards applied to the "male and female" of the other species of the animal creation... the term... was afterwards applied to anything designed prepared for union with another object
HE STILL ISN'T DONE!
You would think, after such a cogent argument, that Rambam would move on, but he wants to make sure that he clearly conveyed critical concept.
It is old tradition that in Torah studies, an argument or statement repeated is important, and the more frequently it is repeated, the more important it is. Therefore, that Rambam repeats his argument in the same chapter clearly emphasizes the importance of this concept to the rest of his exegesis.
Brothers and Sisters: Form and Matter
Rambam ends the chapter with a powerful metaphor:
It will be easily seen that the Hebrew equivalents for "brother and sister" are likewise treated as homonyms, and used, in a figurative sense, like ish and ishshah.
Rambam wants his reader to understand that this concept of form and matter persists throughout the Universe: if it applies to something as elementary as every living creature and the most basic social unit, then clearly, to see Form and Matter in our Universe makes logical sense.
If Form and Matter characterize our Universe, then, naturally, it is appropriate to use this concept in our Bible study and all aspects of study.