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Why this special treatment for mana-abilities? What are the design implications if they didn't go on the stack?

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up vote 21 down vote accepted

There are three reason I can think of:

  1. It would force arbitrary actions to be reversed.
  2. It would allow spells that haven't been fully cast to be countered.
  3. MTG already gives you enough of opportunities to react.

Forcing arbitrary actions to be reversed

Consider this rule:

601.2 [...] If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the casting of the spell is illegal; the game returns to the moment before that spell started to be cast.

Players can obtain mana in the middle of casting a spell[1]. If mana abilities used the stack, that means players would be able to cast spells in response. If the spell were to become unable to be cast, one would have to rewind all direct and indirect effects of those spells. Someone could have revealed private information, shuffled their library or lost the game as a result. How do you undo those?

Not only that, it would also make it far more likely for a spell to become unable to be cast, something that should never happen.

For example, consider this scenario where responding to mana abilities would be allowed.

  1. Alice only has a single creature on the 'field.
  2. Starts casting Altar's Reap.
  3. She activates a Swamp's ability.
  4. In response, Nancy casts Beacon of Destruction
  5. Nancy kills Alice's creature and shuffles his Beacon of Destruction into his library.
  6. Alice can no longer complete the casting of her spell (because she can't pay the additional cost), so the game rewinds to the start of the casting of the spell. Nancy needs to unshuffle his library to match the game state.

Allowing spells that haven't been fully cast to be countered

The first step of casting a spell[1] is to place it on the stack[2]. A spell is a card on the stack (or a copy of spell), so it could be the legitimate target of a Cancel cast in response to activating mana abilities. That's just too complicated and messy to allow.


Plenty of other opportunities to react

Activating mana abilities is not something someone one normally does for its own sake. It's done as part of something else, such as casting a spell. Allowing this to be interrupted would be detrimental to the game flow and would add confusion. There are plenty of other points at which players can respond.

The earlier problems only surface when activating mana abilities when one would not have priority under the current rules. Could the game allow players to respond to the activation of mana abilities performed when a player has priority? Yes, but it would often be hard to distinguish whether an activation occurred before or during casting. It's far simpler to have mana abilities never use the stack rather that having the player try to figure out whether one can respond to a particular activation or not.


Notes

  1. Or activating an ability. I'm only mentioning spells to keep it simple.
  2. The steps to casting a spell[1] are:
    1. 601.2a) Place card on stack.
    2. 601.2b-d) Make choices, including targets.
    3. 601.2e) Determine total cost.
    4. 601.2f) Activate mana abilities.
    5. 601.2g) Pay the previously determined cost.
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How do you interrupt paying a cost to make it unable to be paid? I was under the impression that this could not be done. – Pow-Ian Sep 27 '13 at 18:00
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@Pow-Ian, When 601.2 says "To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs", it's summarizing the entirety of casting a spell ("To cast a spell is") rather than describing the first the first step. The steps are explicitly stated to follow ("Casting a spell follows the steps listed below, in order"). Paying its cost is the 7th and last step of casting a spell. The steps are: 601.2a) Place card on stack. b-d) Choices are made including targets e) Determine total cost. f) Activate mana abilities g) Pay the previously determined cost. – ikegami Sep 27 '13 at 18:30
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@Pacerier, The sacrifice is part of the cost, and it's illegal to cast a spell without paying its cost. It's not illegal for a target to become invalid (though the spell is countered if all targets become illegal). – ikegami Jul 11 '15 at 16:41
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@ikegami Since you brought up the distinction between using mana abuties when you have priority instead of as part of casting a spell; it does seem that the rules regarding mana abilities and casting spells could have been simplified to say that you can't use mana abilities as part of casting a spell; instead you would have to tap the lands you need before you start casting the spell. Then mana abilities could also use the stack like normal. – GendoIkari Jul 12 '15 at 0:56
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Under rules where it is impossible to interrupt between cost preparation and actual cost paying, it does not matter whether you pay first and then cast or cast first and then pay, since nothing can prevent the one from following the other anyway. But if you consider a change of rules as in the question asked, where something CAN come in between those two it does not make much sense anymore to treat these two events as interchangeable. – hkBst Feb 3 at 12:20

Mana sources could go on the stack the same way as other effects. The main difference would be that you could be interrupted in between preparing the costs to be payed and actually paying the costs for casting a spell. For instance you could use all your mana sources in preparation for casting a spell, but your opponent reacts by using their own mana sources which will resolve before your own, then after the mana hits the opponent's mana pool using it to cast a spell that discards your entire hand. Since your own mana still has not hit your mana pool you are basically inable to do anything. After you have discarded your hand, you will not be able to use the mana that finally enters your mana pool for the spell that is no longer in your hand.

A workaround for this problem would be to collect your mana in small steps instead of all at once. Thus you would be using a single mana source and asking your opponent whether they want to react or not. Then use the second mana source and again ask your opponent, until you have collected enough mana in your mana pool to cast your spell. Asking your opponent for permission for every single mana source gets very tedious, so instead the ability to respond to mana sources was removed.

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I think the premise of your answer needs to be made clearer. "Mana sources could go on the stack" if what? If the rules changed? Also, this answer goes into great detail about the history of mana abilities (how they used to work, how they evolved, and how they work now). I think it covers most of what is said here already. – Rainbolt Feb 3 at 14:10
    
The premise is exactly the original question. I assume the premise and explore its consequences. Those consequences are found to not be an improvement to the game, giving the reason why the rules are not like in the premise. The answer you linked to fails to assume the premise and subsequently fails to explore its consequences, and thus fails to answer the posed question. – hkBst 2 days ago

There is some history behind the decision. Back in the dark ages of MtG rules, before sixth edition, there were a lot of rules that were unintuitive and confusing. With the release of Fifth Edition, a new type of ability was introduced,

Mana Sources: "Mana sources" are a new category of abilities. They include the ability of lands to be tapped for mana and all abilities that provide mana as interrupts. For example, Llanowar Elves's ability is now considered a mana source, but the ability of Ice Cauldron is not. Mana sources may be used whenever desired and may not be interrupted; there is no gap between playing the ability and resolving it. For example, you can't Rust a Mox; only continuous effects can stop a mana source from producing mana.

So, a Mana Source is anything that generated mana. Spells like Dark Ritual from earlier editions can be found with the card type Mana Source. Fifth Edition rules clarified that they may not be interrupted. This was necessary, because before the stack, Interrupts (depreciated, and later reclassified as Instants) were handled in batches

In the beginning, there was the batch. You played a spell, a flurry of "fast effects" were played in response, and then everything (well, everything except interrupts) resolved using the "last in, first out rule," with no chance of playing more spells in the middle. [...]

The timing issues presented by Red Elemental Blast and Blue Elemental Blast (and later, Hydroblast and Pyroblast) were particularly troublesome. Interrupts made sense when they targeted spells, but what happened when they began targeting permanents? Remember, removing the source of a fast effect does not remove the fast effect itself, but what about stopping the fast effect in response to it being played? [...]

Under the current rules, Mark taps his Prodigal Sorcerer in an attempt to deal 1 damage to Randy. In response, Randy plays Red Elemental Blast, targeting the Prodigal Sorcerer. This destroys the Sorcerer, but not before its "ping" effect goes onto the stack. That 1 damage is still dealt to Randy. Now, under the earliest Magic rules, Mark would announce the tapping of his Sorcerer, but Randy could say, "Hold on a minute! I want to interrupt your effect." This would stop the game, target the Sorcerer with Randy's Red Elemental Blast, and prevent the damage from ever being dealt. This created huge amounts of confusion over timing: The active player was supposed to have priority each turn, but interrupts being used this way seemed to contradict the rule. This problem, however, was corrected before interrupts were done away with entirely. The ruling said that if you targeted a permanent with an interrupt, the interrupt was played "as an instant."

So, under the pre-fifth edition rules, you could kill a Llanowar Elves or Interdict a land tapping for mana to prevent a player from generating enough mana to play a spell, and the game would have to backup to the point of the illegal action of casting a spell you couldn't pay the cost for. Sixth Edition rules and the stack did away with all this confusion. There are no longer batches and special timing rules for Interrupts/Instants, they all go on the Stack. Abilities that generate mana don't go on the stack, amd therefore cannot be responded to, removing the necessity to specify on the cards that they cannot target mana abilities.

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I think the simplest answer is that mana abilities are generally used while casting a spell to pay a cost. If they had to be put on the stack, then they could never be used while casting a spell because you would be taking another action before a previous action is complete.

601.2. To cast a spell is to take it from where it is (usually the hand), put it on the stack, and pay its costs, so that it will eventually resolve and have its effect. Casting a spell follows the steps listed below, in order. If, at any point during the casting of a spell, a player is unable to comply with any of the steps listed below, the casting of the spell is illegal; the game returns to the moment before that spell started to be cast (see rule 717, "Handling Illegal Actions"). Announcements and payments can't be altered after they've been made.

...

601.2h Once the steps described in 601.2a-g are completed, the spell becomes cast. Any abilities that trigger when a spell is cast or put onto the stack trigger at this time. If the spell's controller had priority before casting it, he or she gets priority.

If you follow 601.2 to it's conculsion you see that you could not activate the mana ability if it was not treated differently because you don't have priority while you are 'casting' a spell.

Yes you had priority when you started to cast the spell and you will have it again when you are done casting the spell, but while you are 'casting' it, (Paying its cost, choosing its targets etc) there is no passing of priority so no abilities or other spells can be used/cast.

So if it was treated like a normal ability, which you also can not activate while casting a spell, then you could never use them to pay a cost while casting a spell. they would always have to be used before you started paying the cost.

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It's silly to assume the game would allow you to activate mana abilities without allowing them to resolve (give you priority)! This is a perfectly correct yet perfectly useless answer. If you argument that we can't have X because we don't have necessary dependency Y, it just shifts the question to "Why can't we have Y?". But you left that unanswered. – ikegami Sep 28 '13 at 2:41

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