Bryan Register should not have conceded defeat, for I am afraid that Bohemian misunderstood the nature of the quote from ArM5 that he quoted. I will develop a problem for such a reading of the quote, which if I am right speels serious trouble for the quote. However, given David Chart's background in HPS, I doubt that the reading supported by Bohemian is what is intended by the author of that particular text. Thus, I also intend to supply a more charitable reading of the quote from page 79, that allows a more consistent cosmology.
First, a bit about my interest: A friend of mine, Kryslin, who posted on this site that gender was accidental. I tend to agree with this assertion with some qualification. Iw ill elaborate later, but first some background.
The essential nature of something is that which can not be changed except by maybe God (and maybe Hermetic Magic using Raw Vis to overwrite the essential nature perhaps... but I do not wish to debate this issue... the forum seems to be doing fine on that). For example, what is the essential nature of a cat? Is it that it should be four -footed? No, because if that were the case, anyone could change the essential nature of the cat by simply cutting of a leg? Is it that it is a particular color, or has fur, or has whiskers? Once again these are features that could be changed, and some of these features must hold for all cats, if it is the essential nature. In other words, it is the 'cat-ness' that is the essential nature of the cat. So a magus could through the use of a Mu An (with perhaps mantem and corpus requisites) change the animal into a person, but that 'person' would still be a cat essentially(ie have the soul(s) of cat-- see below).
It is a little unclear what Aristotle thought might makle up a thing's essential nature. He give us what might be guidelines, but he does not solidly commit necessarily to any concrete essential nature other than man is a rational animal (even that is a controversial with modern scholars, but the medieval Scholastics had a much more settled account which I delineate here). Part of what Aristotle half-heartedly termed might be a part of a thing's essential nature was its souls. Basically, he postulated that every living thing had a soul (Psuche -- no greek font so forgive the transliteration). Plants have a nutritive soul, which gives them the power to grow and reproduce. Most Animals also have the nutritive soul, but also have two additional souls: the intelligible soul which allows them to perceive, and the motive soul which allows them to move. Humans have those three souls plus the additional rational soul. Thus, humans are rational animals. From this concept of souls, we begin to see what might be included in a living things essential nature. This is clearly what is intended by the writers of ArM5:
"All humans beings are essentially human; mortal creatures with reason, senses, and the ability to move or reproduce" (p.79). Or they have the four souls (psuche) suggested by Aristotle (De Anime).
Of course, the idea that animals and plants have souls is contrary to Christian doctrine, so the Scholastics accepted this (through their lens of understanding Aristotle through Augustine and Aquinas) as a characterization of a thing's essntial nature. (Christians tacked on the soul as that which is capable of salvation as a fifth part of a human's essential nature... note that the Faeries would have four of the five essential natures that humans have.) So returning to our cat example, the essential nature of a cat is that it has the abilities to grow, reproduce, sense, and move plus a little something else that differentiates it from other things that have those four abilities. (I will stay away from artifacts for that is a much more tricky issue, ironically.)
One should note that part of a animal's essential nature is that it can live (grow, reproduce, sense, and move). A dead cat is no longer a cat. It is a carcass. So, while a thing;s essential nature cannot be changed by anyone but God, an essential nature can be destroyed. (For those who are philosophically minded you should readily note that this seems to be a problem for Aristotle that a cat (or Socrates) goes out of existence upon death and is replaced by a carcass. This is called the elusive matter paradox, and it is puzzle still today, but in Ars it is merely a mystery that has not been revealed to mankind yet.) Now, if we were to say that cat is essentially a cat, we would be correct. However, this does not entail that a cat must necessarily (ie by the will of God) always be a cat. At some time the cat must perish and become a carcass, thus it says that a living cat is a cat and will be so until it is no longer a cat.
Now on p. 79 it states that "Men are essentially male and women essentially female..." This is true in the same way as it is true that a cat is essentially a cat. As long as a man is a man, he is male. But what happens when a man is castrated or emasculated(physically not figuratively)? In the medieval paradigm, they did not have genetics, so they did not judge gender based upon x and y chromosomes. They based it upon the possession of the right equipment. How else could they determine gender? Furthermore castrati and eunuchs were not considered male, they were consdiered neuter. If we were to read the above-mentioned quote as Bohemian has suggested that it be read, then no power on earth could change that a man would no longer be a man. Thus, we would have to further conclude that no power on earth could detach a man's equipment without killing him and having him cease to exist (to be replaced by a corpse... although admittedly, he might wish that he were dead). Thus we have to assume that while being a man or woman makes one essentially male or female respectively, being a man or a woman is not the essential nature of any particular man or woman. (To assume so is fallacious... but understandably so since it is common mistake given propositions and utterances like the one found on p. 70)
For example, Pallium Tenebrae Guernici is a man. If he were attacked by the vicious Genus Mutatrix and changed into a (not quite essentially)woman he would no longer be essentially man, but s/he still would be essentially Pallium. The magic would not have violated the rule of essential nature in doing so... because it did not affect Pallium's essential nature, but eliminated his being a man (and it could be made permanent according to my understanding of ritual magic).
Complication 1: What could not be done is to alter that part of Pallium's nature that allows him to grow or reproduce. In other words, even if Genus Mutatrix changed him into a woman, s/he would not be able to give birth and would therefore be a barren female.
Complication 2: If Pallium was a transvestite, (from character creation - see page 80) would s/he then try to dress as the other sex? Or would it be that s/he was a female soul in a man's body and that the transformation was part of God's plan... and therfore the flaw would be replaced by something similar to the raised from the dead flaw? Would Such a pallium have been infertile from the beginning but upon the transformation be made fertile? These are interesting questions that I will not attempt to answer, as I* feel that such interesting questions should be the domain of the troupe.
Given the problem of castration and the neuter gender, I do not believe that the strong reading of the quote on p. 79 is justified. While a man is essentially male, Being male is not a part of any particular man's essential nature. This is a better reading, one that is more in keeeping with the intent of Ars and Aristotle.