In the Magical healing thread I mentioned that Muto seemed more appropriate for non-permanent stat changes. I don't know if there are official guidelines for this in a supplement I don't have (all of them ) but if not, here are my suggestions.
MuAn/Co/Me base levels
5 Change one stat to the average of a creature of that type (0 for humans)
10 change one stat to +/-1 from the average of a creature of that type
15 change one stat to +/-2 from the average of a creature of that type
20 change one stat to +/-3 from the average of a creature of that type
30 change one stat to +/-4 from the average of a creature of that type
40 change one stat to +/-5 from the average of a creature of that type
50 change one stat to +/-6 from the average of a creature of that type
Physical stat changes will have visible physical effects - more or less musculature for STR, litheness and smooth movement or gangly clumsiness for DEX, twitchiness or drowsiness for QUI and ruddy leathery skin or pallor for STA.
Characteristics of humans are part of humans, and decreasing them is IMO perdo, increasing them Creo.
Muto could do something unnatural, like giving giant strengthfor example a score of +6 or +7 in Strength. For guidelines i'd use something based on the one for soak. So to have the unnatural strength of a giant a spell base 30. It would be almost like becoming a giant, but without the size upgrade who can be problematic.
Not unless you use Muto , because you can only get to -05 by having Poor (Characteristic) on page 58.
This would make the Flaw part of your Essential Nature.
If virtues and flaws are necessarily essential nature and not merely a somewhat balanced game mechanic to add detail and variety to the character as they are when they enter play. After all there is a spell level guideline to take someone from a +3 stat to a +4 stat but to start play with a +4 stat you'd need a virtue. There are spells to cure lameness and missing limbs but at the start of play those conditions are flaws - are injuries that happen before 1220 supernaturally different to those that happen later? I think not, I think characters are malleable and magic and mundane effort and happenstance can all change them.
Well i think the "characteristics" virtues and flaws are not related to essential nature, since the essential nature of human is from -5 and +5 (say what creo say about more than +6 in a characteristic not being achievable by creo since it's out of human nature).
Other flaws and virtues (lame, blind) are related to essential nature, since can't be permanently recovered with creo.
Muto, not changing essential nature permanently but only temporarly, is perfectly usable.
You are without hands? Muto yourself and create 2 or 4 if you want.
Agreed that, for visible unnatural characteristics, muto should work.
Maybe at a tradeof like giant size: Give +2 to a stat, -1 to another, with a visible effect to boot (like huge muscles and all)
I tend to be conservative on this.
To me, +3/-3 is the normal range of human abilities, with the more extreme scores, only possible through virtues and flaws, being more in the domain of the mythical. Like those heroes from the trojan war: They ain't superhuman throwing rocks left and right, but they ain't normal either (and in fact, most of them have divine blood).
Or take it like an action hero movie: No one could endure as much punishment as, say, Bruce Willis does in Die Hard, but it's still theorically possible for a human. Barely believable.
Normal humans don't have a strength at +4 or +5. Heroes do. People born from giants do.
AFAIK (Serf's Parma), that ain't supported/infirmed by any rule, but I think this gives more flavor to those characters, make them more special.
It will be pretty wierd when our Doctor-Magi can cure a rich NPC who was club-footed from birth but not their own autocrat who had his foot run over by a cart 4 years ago and their excuse is "essential nature".
Oh granted, from a metagame standpoint fixing a flaw should be something difficult that requires a story to resolve rather than something that is done off screen as soon as you have the time and resources. However, I think it is for fate, in the form of SG and player, to provide the complications rather than decreeing that the attempt runs into an inconsistent limit of essential nature with no in game logic.
In any case, a metagame protection of virtues and flaws as long term parts of the character does not preclude them being temporarily set aside by suitable magic or in game effort. That's what magic and technology do - they allow humans to transcend the limited capabilities of the naked, hairless person alone.
Overcoming birth defects with magic (if only temproraily) could be an interesting line in Hermetic Research as breaking or even just bending the limit of essential nature is quite an interesting one to break.
but with any improvement the stat should be Creo as it is dedicated to the improvement of a thing and the moving it to a more perfect form. Muto merely changes one thing for another. Muto cannot enhance but could (potentially) swap out club foot for another flaw.
) but if not, here are my suggestions.