This question is inspired by discussion here, about how Heighten Spell works with Reserve Feats. The question that I am asking here deals with whether or not players can cast spells using spell slots for spell levels that they don't have access to yet.
I feel like an example would explain this concept best.
Take Xylitol, the first-level wizard with an intelligence of 18. He has 5 0th-level spells per day and 3 1st level spells per day from his class. He also has 1 bonus 1st level spell per day from his high intelligence. The Ability Modifiers and Bonus Spells table says that he also has 1 bonus spell of each 2nd, 3rd, and 4th levels.
The question is this: Can Xylitol cast 1st level spells using the bonus slots for his 2nd, 3rd, and 4th level spells? He clearly can't cast spells that are inherently higher level than 1st, since he can't learn them, but can he cast 1st level spells using bonus 2nd, 3rd, and 4th level slots?
This has come up on these boards before, here. What I'm looking for here is a clear rules citation that clarifies this one way or the other. The wording in the section on ability scores makes it quite clear that characters cannot cast spells that are actually higher level than their class allows them to cast, but it is not clear that these higher level slots are barred until the appropriate spell level is unlocked. If a character has access to a spell level that they can't use because their ability score isn't high enough, they can freely use those slots for lower level spells, so why can't they use their bonus spell slots for lower level spells?
As KRyan's answer says, the Basics section covers this. Based on questions and comments both here and elsewhere, that section seems to be worded ambiguously enough that it's caused confusion. I'd like to find another, more clear wording to this rule if at all possible.
Answers from either D&D 3.5 or Pathfinder are acceptable, but please note which one your answer applies to.
N.B. I'd like to note that I personally don't feel that players should have access to higher level slots like this. There are a large number of locations where it's heavily implied that this is not the case, but I feel that finding a clear rules citation here would be helpful. I can't find one that is clear for either interpretation here.