Because Heighten Spell is limited only by the highest level spell you can cast then if you are claiming that you are granted the ability to cast X level spells simply by heightening a lower level known spell to that level and having a high enough attribute score to cast that level spell, then you are also claiming that a 1st level spellcaster can cast 9th level spells. There are a few Metamagic feats that you could take at 1st level, and heighten spell only requires you to know any other metamagic feat. There are various ways to get multiple 1st level feats, so it's entirely possible to have Heighten Spell at level 1. Since your only limited by your ability to cast spells of higher levels then you could by your logic heighten a 1st level spell all the way to 9th level and claim you have the ability to cast 9th level spells thus meeting the requirement for any spell level based feat & prC at level 1.
Oh, lord. You've set up a lovely straw-man there. Bravo.
No, this is not my argument. Nor is that the limitation of Heighten. The limitation of Heighten is, generally, determined by your highest level spell slot. If I have a 2nd level slot, and I use Heighten to cast Magic Missile out of that 2nd level slot. By doing so, I am casting Magic Missile
as a 2nd level spell. However, if that 2nd level slot is the highest slot I have available, then I cannot arbitrarily Heighten Magic Missile up to 9th level.
However, let's say that I do not have a 2nd level slot. What I DO have, however, is Earth Spell. With this set-up, let's say that I cast Prestidigitation, Heightened into a 1st level slot. At this point, I am casting Prestidigitation
as a 1st level spell. Earth Spell then kicks in, and Heightens the spell one level further. Thus, I am casting Prestidigitation
as a 2nd level spell. This is despite the fact that I do not have a 2nd level slot.
Frankly, I don't care that you disagree with this interpretation, faeryn. You are not, objectively speaking, right.
I apologize if my alternative understanding of the interaction of these feats has somehow offended you. However, I refuse to engage with you any further on this particular issue (at least within this thread).
If you would like to create a new thread for this particular issue, then please, by all means do so. We've already derailed this thread thoroughly enough.