One of the earliest controversies I can remember from Ars fandom is this: given that magic can’t affect your essential nature, can it change your gender?
The generally given answer is no, and this is because the way gender is affected by magic in other roleplaying games is so terribly, terribly juvenile. Basically being transformed into a woman is seen as a shaming, disempowering, embarrassing thing and so its treated as a practical joke played on male characters. Being a woman is a curse they need to fight and eventually be cured of.
Very early the history of Ars Magica the writers indicated that the Gift was found equally in male and female children and therefore the Order of Hermes embraced equality. Strangely the Founders were and not split 50/50, but you could claim that this was because they were descended from a Roman priesthood, with a patriarchal cultural bias. So magic can’t change your gender because it is essential to your character.
This is rather strange though, because magic can do so much else that isn’t prevented by the Limit of Essential Nature. It can make you older. It can change you, temporarily, into a wolf. It can cut off your arm. These changes are, apparently, less essential than your gender. It can’t make you younger, because mortality is inherent in the human condition, but it can slow the rate at which you age.
If gender is essential, how much resistance to magic does this grant? Your soul’s freedom is essential. You can’t sin under influence of magic. You can’t sell your soul under the influence of magic. Things you are forced to do, you cannot be made morally culpable for by magical means. If your gender is protected in the same way, does it protect the expressions of your gender? I’m not talking about the most overt expressions either. I’m male, so I have androgenic forearm hair. As that is a secondary sexual characteristic, does it have resistance to magical destruction?
The Church had to deal with this issue because intersex people exist. In the modern United States, about 1.7% of babies are born in some sense intersex. The Church, facing similar numbers, ruled that if a person could function more ably in one gender than in the other, that was their gender. The point was to force people to pick one role and keep to it, rather than switching.
There are all kinds of weird writings at the time indicating that, physiologically, sex was more permeable than it than it is in our world. Women were counselled against strident exercise, warming foods and hot baths, because these would affect their fertility and, in extreme cases, cause them to change into men. Despite these writings, there is no sign that children were prescribed warming as a treatment to change a female heir into a male one: Kings without sons, like Henry the VIIIth for example, never fed their daughters pepper, submerge them in warm baths and force the to go hurdling, in the hope they will become boys.
So, in Ars Magica, sex is often said to be immutable, because other systems handle this so badly, and because permanent changes to the mind of the character are seen as disempowering to the player. I’d argue that in Arrs Magica we have a mechanism which allows us to sidestep these issues: mystery cult initiation.
Cult initiation means a character can change deeply ingrained features, but only with effort, so that trivial switching is reduced. If, however, you want your character to undergo a metamorphosis, and spiritually transcend their limiting physical realities, this is possible. This means a transgender character could undergo ritual initiations which allowed a change of sex. Certain ancient Greeks had this done to them by faeries: Tiresias, a blind seer, was changed into a woman by a goddess, and lived as one for many years. This was seen as the source of his wisdom. Mystery cult initiation allows a character to change gender or race, and know the things only people in those communities know. This allows us to pass tangentially through real world religions that see twinspirited people as shamanistic: living on the margin of society, protecting it from hostile spiritual forces.
So to conclude, I understand why early Ars Magica fandom thought male characters could not become female through Hermetic magic, but I’d like to suggest that the later, Mystery Cult, mechanic gives the player enough control over the process that it becomes a roleplaying challenge rather than a childish joke or punishment. This provides a loophole, which some troupes may wish to explore in their sagas.