Yeah, I posted about it during the summer some time. How it works is that every spell has a minimum caster level.
Says who?
The PHB, page 172 under Caster Level.
Varies from class to class and character to character. What's the minimum caster level to cast lesser restoration? Or irresistible dance?
Depends on base class. For Lesser Restoration it's caster level 3 for a Cleric, and probably 4 for a Favored Soul, but I'd need to look at the class. In order to determine the minimum caster level first find what level the spell is (Level 2 for Clerics) then see when that base class would naturally learn level 2 spells. For a cleric, a caster level of 3 is needed at minimum.
So, instead of each spell having a definite minimum level built into, each spell has a relative minimum depending on who is casting the spell.
It also varies from character to character of the same class (archivist, clerics with domains, et cetera). A beguiler using versatile spellcaster + heighten spellcaster has a lower minimum CL for casting his second level spells than an equivalent wizard, or a beguiler without the combination.
IIRC, that section is about reducing the power level fo the spell anyway, so it doesn't come up if you're just casting it.
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I've never said it was a good system, but you're right, it does vary. Spells are not an absolute system, but a relative system. I'm not familiar with archivists, but for the Beguiler I'm assuming you're talking about doing this, please correct if I'm wrong:
You use versatile spellcaster + heighten spell to be able to cast 2nd level spells while you are below level 4. Let's assume for some reason they can at level 1 just for simplicity. He does this trick and sees eternity to gain knowledge of 2nd level spells. So far so good. However, the minimum CL does not change to 1 for those 2nd level spells. They still require CL 4 because that is when a Beguiler would naturally learn them. All that Versatile Spellcaster + Heighten spell is doing for him is giving knowledge of those second level spells.
That same argument would say that because a Wizard is able to write down a spell into his spell book that the minimum CL for that spell has changed. It has not.
Remember that all spells require CL, Slot, and Knowledge. This doesn't change for Clerics with domains.
Domains are different, but follow the same principals. The big one is that that require special slot, a domain slot. If they were a gun, they require a super awesome barrel because domains are super awesome. Let's say there's the Awesome Domain that lets a Cleric cast Mass Lesser Vigor at level 1 with a domain slot. For that individual domain, the minimum CL is 1. Normally Mass Lesser Vigor is a level 3 spell, so it requires a minimum 5 for the CL. So to cast Mass Lessor Vigor as a domain spell, it's CL 1. To cast normally, it's CL 5.
Do I believe this system is really stupid? Yes. Very stupid. Will getting spells before your level be banned or get a book thrown at you? Yes. But this is the only consistent way that spells work across all classes.
An extension of what I'm saying is that whenever a caster levels up they receive the following things. A caster level, a spell slot (if applicable), and knowledge of the spell in some way. The system knows whether or not to grant you a different level of spell based on the CL relative to your caster level. Each class maintains a seperate list with a seperate minimum requirement for each and every spell. So let's say we have the jack of all trades, but really sucks at everything caster of Wizard 5/Sorcerer 5/Cleric 5/Druid 5
This character has 5 different spell lists. Wizard, Sorcerer, Cleric, Druid, and Domain. Each list uses their own seperate spell slots. And to cast a spell from a class, each class must have a minimum spell level. In other words, each list is independent of each other. A Wizard spell cannot be put in a Sorcerer spell slot unless the Sorcerer list also has knowledge of that spell. A Cleric cannot put a domain spell in a Druid spell slot. A Sorcerer Glitterdust needs a minimum CL 4 while the Wizard one needs a CL 3. Even if they're from the same character. Each list is independent.
Sorry if my explanation or writing got a little confusing.