A mage's spellbook is like a book of recipes. And like most medieval recipes, it's written more as a reminder to the mage than as a guide to someone else. So when the recipe says the equivalent of "put near the top of a hot oven and bake until done," the mage trying to decipher that recipe is going to be a bit challenged.
The wizards' Spellcraft skill includes the skill of deciphering such a recipe. If you fail to decipher a recipe, you really won't be able to do so unless you learn more about the deciphering of recipes. You may be able to learn that technique through other methods, like getting the original creator to show you how it's done.
Having the original caster show you how a spell is cast (prepared then cast, in the D&D spell model) would give you a significant bonus on the Spellcraft check for learning a spell. But it still would not be a sure thing, because the two of you might not realize some critical detail that needs to be communicated.
Sorcerers don't use recipes. They're like the cooks who can make wonderful items, but can't read a recipe or effectively imitate another cook.
Classes who don't cast arcane spells are basically unaware of the whole kitchen of arcane magic.
To continue this metaphor to magic items:
- Potions are like wholly prepared foods. They need no kitchen smarts; just unwrap and eat. Other things like rings fit in this category too.
- Wands are like instant hot chocolate or coffee--anyone with a bit of kitchen familiarity can trigger them. This is why sorcerers can use wands.
- Scrolls are like the 'meal-in-a-box' products. (There are some that are like Hamburger Helper, but include all the ingredients required. I can't think of a product name at the moment, though.) It doesn't take too much cooking skill to make one of these (which is why sorcerers can use scrolls). These products also include the property that their ingredients are evident enough that it's conceivable for someone clueful in deciphering recipes to be able to reconstruct an equivalent recipe from the set of provided ingredients, which is why wizards can learn spells from scrolls they find. And it's possible to prepare the recipe from the box even if you can't reconstruct the recipe, which is true for scrolls too.
Hey. This all kind of makes sense. Imagine that. This doesn't always happen with D&D.
Some corollaries of this metaphor:
1. There's no particular connection in this metaphor between those three types of food products and the three types of magic items. So maybe I will have things that behave like potions but aren't flasks of liquid to be drunk, or things that behave like scrolls but aren't portable rolls of paper. (Imagine a scroll-effect item that someone has thoughtfully carved on a large boulder...)
2. Someone who was trying to write for the benefit of others could be extra-clear in their recipe-writing, making spells that were easier to decipher. This, though, is not the normal case.
3. If you failed a Spellcraft roll to decipher another caster's book or spell, having that material would still make it much easier to research that spell than doing research without that guidance.
4. Research wizards will generate a lot of notes on "recipes" that went wrong, and they'll only copy the ones that went right into their spellbooks. This means that wizards' libraries will could have a lot of notebooks and papers without being wall-to-wall spellbooks.
I rather like this metaphor. What do y'all think of it?