(This applies to D&D (all editions except 4e) as well as all CRPGs that copy this aspect of the magic system, including Wizardry 1-5 and Final Fantasy 1 and 3.)

One thing I have never really understood is the reasoning behind segregating magic resources by spell level. This fact has really strange consequences; for instance, if a character uses up her first level spells on Cure Light Wounds, why does she still have the energy to cast Cure Serious Wounds (which is objectively a more powerful spell)? What is it about Bless that makes it share its resource with Cure Light Wounds, but not Cure Serious Wounds?

It would really make more sense if all spells used the same resource (like in every other Final Fantasy game except 8), or if resources were segregated based on type or element rather than power (like in Wizardry 6-8), or by type of ability (SaGa Frontier separates the resources for physical and magical attacks).

There's also the problem that segregating resources by level doesn't really make much sense from an in-world perspective; again, why do Bless and CLW share the same resource, but CSW does not?

There's also a balancing issue at high levels; characters can use their high level spells without sacrificing the ability to use low level spells. While I can understand using low level spells to conserve high level spells, the reverse doesn't really make much sense.

There's also the fact that, depending on the game, some spell levels might not be useful. For example, in Final Fantasy (NES version), a Red Mage has little use for 2nd level spells; there's no healing spell (unlike 1st and 3rd level), ICE is only single target and not that useful when you have a decent physical attack, TMPR doesn't work properly (in remakes, this spell was fixed and became the best spell of this level by far and one of the best spells in the game), and the other spells aren't very useful.

(Note that whether the resources have to be allocated in advance (which 2e AD&D calls "memorizing" (a term that doesn't make much sense) and 3e calls "preparing" (which makes more sense) is orthogonal to how casting resources are segregated; we could have a system where your resources are allocated in advance but all use the same pool; Terranigma's magic system is sort of like this.)

Your very first sentence references Wizardry and Final Fantasy with nary a mention of Baldur's Gate. You then go on referencing various games *except* the game that this forum is all about. This is a game specific forum, not a general D&D / RPG / JRPG forum.

Pass.

Anyone have anything to say about this topic?

(This post here so that this topic would appear in the "topics I've participated in" list.)

Let me try to explain it using an analogy. And I just made this analogy up right now, so I’m sure there’s some flaws to it.

From your questions, you’re thinking of spell energy as a “liquid” and using the logic: if I have enough liquid to cast one “two-liter” spell then I should also be able to cast two “one-liter” spells.

But in the 2nd Edition AD&D magic system (and, by extension: Baldur’s Gate, Icewind Dale, etc.), your spell slots are like a container of a specific shape (circle, triangle, square, pentagon) – and spells are solid objects which also have those shape. Thus, you can’t place a weaker spell into a stronger slot because their energies are of different patterns (or “shapes”). However, as with many aspects of 2nd Edition – the logic of the rules breaks down pretty quickly if you take a critical look at it.

Third Edition starts to address this by allowing meta-magic feats – enhancing the power of a spell and altering its energy pattern to fit into a different slot. And Fifth Edition goes even further to basically say: “yes, even though you’re wasting some of the space, you can shove a triangle into a square slot if you want to” (i.e. spell slots can be used to cast any spell of that level or lower).

Actually, third edition lets you put low level spells into higher level spell slots without needing metamagic feats; it's one of the rules that is typically not implemented in CRPGs (though the GOG version of Temple of Elemental Evil *does* implement this rule; even a Sorcerer who is out of low level spell slots can cast low level spells if there are higher spell slots available (and those higher level spell slots do get used up if you do this).

For wizards, the reasoning is that they have to imprint specific patterns on their brain.
For clerics, might be the same, or they pray for specific spells, or make your own reason - that's why it's called roleplaying.

IIRC, the conceit of Vancian Magic is that you're memorizing or preparing specific incantations that are erased from your mind as soon as you use them. I haven't read Jack Vance's work, so I don't know how he originally explained it, but I'm also not really in the business of picking apart the logic of magic systems in tabletop RPGs. It's one of those weird anachronisms that has stuck with D&D throughout the years, but if the reaction to 4th edition was any indication, it's also too iconic to ever really get rid of.

If we take D&D into consideration, the reason is that magic does not use resource but memorization. After having played Diablo 2 for over a thousand hours around 2000, that memory system felt really strange at first to me too. But the more I thought about it, the more it made sense.

D&D does not use a resource as mana but a memory slot in your brain.

You have a wizard book. You write down the spells you learn there. However, in the heat of the fight, you do not want to open your book, try to find the correct page and read out loud the spell while someone is hitting you with a sword. Instead, you memorize some of them and utter those words quickly while you fight.

Why spell levels?

As we advance our knowledge in a topic, our knowledge/memory becomes like a pyramid. We extend our knowledge about the basics while we have the more narrow understanding of the more complex part.

Thus, we can memorize more of easier spells (level 1) and less for more complex spells. This also has a capacity/limit of course.

But why can a cleric use a level 3 healing spell when s/he ran out of Level 1 healing spell?

Because they are not same spells. You utter different words while casting those spells. Now, remember that our resource is our memory, not mana and when we cast a level 1 and level 3 healing spells, the resource is not the same.

All good but why can't I cast low-level spells using my high-level memory slots?

I am not clear on that topic myself. I think that's more about the complexity of the game rather than logic.

I decided to do a little internet search, and I found some evidence that the Dying Earth series doesn't segregate spells by level, but instead has a sort of pool of magic capacity with which to memorize spells.

There is mention of a character who can memorize 4 major spells or 6 minor spells. If magic worked just like AD&D, the character would be able to memorize 4 major spells *and* 6 minor spells, with the major and minor spells being completely separate slots.

Also, Vancian magic and level segregation are different topics; aside from the DE series having Vancian magic without level segregation, there are some games with level segregation but not Vancian magic, like the Wizardry and FF games I mentioned above, or even the Sorcerer class in BG2. Except that, in AD&D (1e and 2e), at some levels a Cleric with 19 or more Wisdom can memorize more 4th level spells than 3rd level. (There's also some anomalies in classic Wizardry due to how spell points are allocated (1 per spell if you aren't both a high enough level and in a class that normally learns the spell, and FF3 DS has a couple class that get more casts of 5th level spells than of any lower levels.)

(As a side note, the healing spells in AD&D 1e and 2e don't seem really well thought out; there's a 2-level gap with no healing spells (with the only healing spell available being so weak and unreliable* that it's only useful because resting doesn't fully heal you, creating busywork when it comes to healing). Plus, there's the huge gap in power between Cure Critical Wounds (rather weak) and Heal (extremely powerful, though still only single target).)

* Note that many off the IE games, including BG2 but not BG1, fix the issue by making healing spells always heal the maximum amount, and BG2 and IWD both introduced a new healing spell (though not the same one!) that sits in between CLW and CSW, reducing the gap to a more reasonable 1-level gap.

Magic in D&D comes from something called the weave. Magic is also a language. Not much is known about it except that it comes from Mystra. Throughout the multiverse there are spots where magic cannot be cast. The language is unique in that when a mortal casts a spell it is erased from his memory. In Abeir the weave is absent entirely, and only supernatural creatures such as dragons can use magic.

Cure light wounds and cure serious wounds have similar effects but the wording in the language is different, which is why when you cast one the other doesn't go away. Creatures like the pit fiend can cast fireball at will and I'm not really sure why that is.

Edit: Divine spells have different mechanics to them, but like arcane magic not much is known about the technicalities. Divine spells increase and decrease in power depending on where you are in the multiverse. Power coming from a lawful good deity will be diluted in the lower planes. It is common for a paladin or a cleric to go to the blood war to start a crusade. Their powers fail and they end up dying pretty quick.

You mention the source of magic in D&D, but you are really describing how magic works in one specific campaign setting. In other D&D settings, magic has a different source (Dark Sun is a good example; interestingly enough, the Dark Sun CRPGs use spontaneous (rather than Vancian) casting, but still segregate spells by level).

Also, I would expect CLW to have more in common with CSW than with Bless, yet CLW and Bless share a resource, but CSW uses a completely different resource. That's why the level segregation makes no sense to me.

You seem to have a hard time understanding that there is no "source" mechanism. Saying it over and over again won't make it true.

What do you mean by a "source" mechanism?

What are you after with your questions? Do you simply want to better understand why the magic system was made as it was back in the day? Do you want to try to feel more connection to the game world, making it more logical and thus more relatable and enjoyable? Do you just want to say it's stupid? Do you want to try to make your own magic system, or do you want to see where video games should go in the future?

What is it you want to understand?

Well, I could see it going multiple ways; in fact, any of the reasons could be the sort of thing that I might be after and want to understand.

Specifically, I am wondering how:
* Such level segregation makes sense in the game world, as such worlds don't generally have "level" as an in-world concept (exceptions include fourth-wall breaking in games like Disgaea and the Final Fantasy series having strange spells like Level 5 Death (instant death if target's level is a multiple of 5)).
* Such level segregation makes sense from a game design perspective. Does it really make sense that different powers of spells should use entirely different resources (instead of using different amounts of the same resources)? Does it really make sense to keep track of 9 different resources for high level casters? (I suspect that this system might be part of the reason D&D campaigns often stick to low (typically single digit) levels.)