Hypersmurf said:
It's below the minimum level at which he can cast it!
-Hyp.
So? I'm not arguing about the level at which he can cast it, I'm talking about the effective level which is possible for the spell, which is any number, whatsoever, that the rules assign to the effective caster level.
As an example, a 5th level cleric can't choose to cast a spell at 6th level. Aha! But if it's a Good spell and he has the Good domain, the spell has an effective caster level of 6. Now, our cleric friend has the option of casting it at level 5, since his ability raises his effective caster level, and does not simply modify it at the end. But it's very easy to prove that casters can cast spells that they can't cast, in the limited sense you are using.
It's below the minimum level at which he can choose to cast it. But I don't see any rule anywhere that says it's below the minimum level at which it can be cast. As noted already, a 5d6 fireball is possible for a wizard but not a sorcerer. Therefore, a caster level of any arbitrarily low level will not cause it to fail.
A caster cannot choose a caster level lower than, say 5, but there is nothing preventing his
effective caster level from being modified. He simply cannot choose a level below that when he first decides which spell to cast and determines if he can legally cast it.