That's my opinion.
My point was that if changing any property counts as changing an essential property, then we've contradicted half the spells in the book, so we should recognize the difference between essence and accident. My argument for this distinction is that, if the writers are going to say that Aristotle is the authority on natural philosophy for magi, and the magi, presumably using the vocabulary they know, will respect something that they call the limit of essential nature, that limit should involve something recognizable as Aristotle's essences and natures.
But now my GM (who is where I got that changing essential nature requires vis) tells me that only certain long-term changes to essential nature require vis. What the 'maintaining' that one must do to sustain a change in essential nature comes to, I have no idea.
I don't understand the book's use of the term 'essential nature,' but I apprehend now that it has nothing to do with essences or natures. And I don't understand the limit of essential nature, since it doesn't seem to limit anything. I'm borderline to giving up.
Okay, guys, thanks very much for this discussion.
This is the limit of essential nature, as I understand it:
You can't make a permanent change to... well, anything. A thing, any thing at all, is what it is what it is. Muto spells, which change an aspect, ANY aspect, of a form are inherently limited in duration. A spell to change something, anything, has to be recast at the end of its duration. At the absolute most, a magus can change something for a year at a time, but will have to recast the spell at the end of that year. Not only that, a spell with a year long duration requires a ritual and the use of vis, and any person under its effects will suffer from warping effects.
Now a creo spell can either bring something from nothing (a loaf of bread or a barrel of wheat), or it can restore a degraded form (a healing spell.) Either way, the limit of essential nature is not transgressed, since in neither case is something being changed into something else.
In the previous edition, I believe that permanent changes could be affected through the use of vis, which may be adding to the confusion. And if I've ever, in real life, said anything that contradicts any of this, I apoligize. It can be rather confusing, and I'm often just straight up wrong. Hell, I could very well be wrong about what I just wrote. I could be wrong about being wrong.
Someone noted that a child would be essensially a child. This is not true, as a child will grow up to be an adult. If you could naturally affect such a change, then it does not defy essensial nature, if you could not naturally affect it, then it does defy EN.
I believe that the essential nature restriction was far more relevant in previous editions of the game where there were spells of perminent duration.
In fifth edition the only application of the essential nature limit that comes to mind is that it prevents some of the characters with the lame flaw from being healed with creo magic (if their deformity is part of their essential nature).
I appreciate this explanation, and I think that what Erik says about permanent duration spells makes it clear what's going on. I note only that the limit of essential nature isn't a limit, and has nothing to do with essences or natures. My error was in thinking that the limit of essential nature might be a limit, of essential nature. 
Well, what confuses me isn't necessarily confusing in any objective sense. You're clear, the book isn't, and it was in trying to force the two of you into clarity and consistency that I got to a problem. Sorry to drag you into my nightmare of rules interpretation! It's not your fault! 
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.
Consider this: muto is the technique of (temporarily) ignoring an essensial nature. So if you need muto to affect a change, then you're affecting essensial nature.
Ok Guys.
After reading through this Post,I got a Question:
First lets assume,that a female Bjornaer gets hit with a mindsupressing Spell (Mind like a Beast or so) while in her Heartshape.
Lets assume said Spell holds for a month, during which she meets a sexually compatible Animal,and get´s Pregnant ?
Whats she getting after recovering, Puppies ?
I am not sure I buy into the 'being a baby is a babies essential nature'. Seems circular.
If one can't effect essential nature, yet a spell can cuase a seed to grow into an Oak. Thus magic can cause a baby to grow into an adult, and therfore would be violating essential nature - which is not the case as the 'Wizard's Oak of No Age' is a pretty classical effect (since way back in 2nd ed I think).
All I am saying is that I suspect one has to be very careful when using the words 'essential nature'.
On a related note - is the mind still just a shadow of the soul, and not part of that immutable, imortal creation?
Unless, LOL, I am talking out of my behind because 5th ed. changed the limits of magic. In which case, please ignore my wild ravings - I must simply be having a Crianomon day 
EDIT: Whoa! five new posts came up just as I was posting. Maybe my questions are pre-answered. Cool.
SECOND EDIT: Wow. Good explanations. Nice to have some help with the paradigm as my knowledge is so often incomplete. It really is hard to divorce one's self from the modern mindset at times.
So what is this about no permament effects in 5th? Man, I need that book. IF only it were in stock ... so hard to wait for my ArM fix.
Not only shouldn't you buy it, you can't: no one was selling it. That was the point of the example. Since no one would ever accept the claim that being a baby is essential to babies, the following argument ought to be persuasive:
If every property that a thing possesses is essential to it, then being a baby is an essential property of babies.
But being a baby is not an essential properties of babies.
Thus, it is not the case that every property that a thing possesses is essential to it.
That matches the basic logical form called modus tollens:
If p is true, then q is true.
q is not true.
Thus, p is not true.
Apologies. Just a turn of phrase in friendly discussion. No offense meant
Thanks for the explanation on logic though, as it's always nice to learn something.
Simple rule: if it can be changed by natural means, then it is not part of EN. If it cannot be changed by natural means, then it is part of EN.
So, female human baby is essensially human and female, but not baby (since any baby will change into an adult given time).
The problem with "if people can';t change it, it is essential nature" as suggested by Ulf, is that there are recorded cases in period books of women spontaneously becoming men, by vigorous exercise that altered the balance of their humours. The change is instantaneous (one girl leaps a fence and comes down a boy).
As to "men are essentially men", I think that's a modern nod to the basic fear of emasculation that so many roleplayers seem to suffer. Thier gender identity is eweak to begin with, because they are, well, pimply and undateable, and so being turned into a girl is a sort of curse, and has homosexual overtones.
Have I gone too far, here?
It's not that you went too far.
But that one can never find size 22 stilletto heels for that formal occasion. 
Timothy> It is worth noting that while some strange magics may alter EN, Hermetic magic may not. So the girl becoming a boy by jumping a fence is probably affected by some other magic.
It was probably a [color=darkblue]+05 Cursed Fence of Sex-Change.
Ulf, the boy becoming a gilr by running and jumping isn't affected by magic at all. This was a medieval cautionary tale, telling other girls that if they did the same, the same might happen to them.
Again, I see you going for magic, when the medieval explanation is far more interesting, because its just so alien. Maleness is not an inherent characteristic that God has sealed on your soul so that it can't be stolen from you by the wicked magi...except in Ars Magica, because some players, whenever we have this debate, seem to think that they'll cease to exist if they don't have a penis.
They go so far as to admit that yes, they can be turned to stone, but it must somehow be -male- stone, or they can be turned into a flowering tree, which strucutrally means they have dozens of uteruses, because flowering trees are mostly hermaphodites, but they can't get past the idea that they must hold onto their maleness, or they are not them anymore, and so it must be essential nature. This time around, the rules agree with them...so far. You are turned into water that's -male- water, and turned into -male- bricks. I mean, what the...?
Actually being human is EN too (excepting Bjornar here) - Thus HERMETIC magic can't PERMANETLY change you into a stone/tree/differnet sex.
As for the Medival tale not refering to so kind of magic (faerie/infernal/whatever) effect... Well, suffice to say in any game I'd run there would be an supernatural explaination, as I still fail to see this following nature...
It is a well-kept secret that proper balance of male and female bricks must be observed in the construction of any building. 