As a preface, the example I'm going to provide uses Panglacial Wurm, because professionals have considered it in Mono Green Tron, a competitive viable deck. This question has implications outside of Panglacial Wurm, including all Future Sight effects.


Stated simply, there are several steps in casting a spell, which you complete in a linear order every time you cast a spell. If a player knows that they cannot, or do not want to, complete one step, is it still legal to complete all steps preceding the step where they intend to break? I realize that this question is functionally irrelevant in almost all cases, but not every case.

For a functional example, Aaron has Expedition Map, Chromatic Sphere, and three lands in play. Aaron sacrifices Expedition Map, and begins searching their library, starting by picking up and looking at just the top card of their library. Aaron sees that it's a card that they'd like to draw, so they separate it noticeably for their opponent, and clarify that it's the top card of their library before continuing. They thumb to a Panglacial Wurm in their library, and say “I'm going to move Panglacial Wurm to the stack. When I can activate mana abilities, I will sacrifice Chromatic Sphere, adding green. Draw this card,” then they point to the card they separated earlier, and don't physically move it into their hand. “I can't pay the total cost of Panglacial Wurm, so it goes back to the part of the library it came from. I can't reverse my Chromatic Sphere draw. I'll continue resolving Expedition Map.”

In the comprehensive rules, 721 handles illegal actions. 721.1 begins, “If a player takes an illegal action or starts to take an action but can’t legally complete it, the entire action is reversed and any payments already made are canceled…” By beginning the steps of casting a spell, without the ability or intention to complete those steps, Aaron has brought the game to an error state, which must be corrected. We already have a prescribed way to correct the game state. What we lack is an answer as to whether or not Aaron has committed an infraction by intentionally bringing the game to this state. Has Aaron “taken an illegal action” or has Aaron “taken an action but can't legally complete it.” All actions that Aaron has taken are legal, with the potential exception of beginning a process he couldn't complete. The question is just that; is beginning the process illegal if completing the process is impossible or not the intention.


I believe the existing comprehensive rules support interpretations in favor both for and against this being an infraction. I don't think there's an unambiguous answer that exists at this moment, which is why I bring it here instead of the floor of a competitive REL event. For what it's worth, while it's a confusing line of play to allow, I don't worry about the harm of this trick being legal. It doesn't seem like it can cause any damage, as just one of many arcane loopholes one can utilize given a deep understanding of the rules, similar to the recent Krark-Clan Ironworks trick that has been discovered by the larger community. I am worried that by designating it illegal, we are setting a precedent where our policy looks into the future to determine if present actions are legal.


PS: I promise I'm not playing stump the chump here. The point of this question isn't some game to see if I can come up with the most convoluted loophole in Magic. This started as some passing idea I had about Future Sight, and when I figured out the implications it had with Panglacial Wurm, I realized that it has the potential for competitive play. Green tron can check for a land or bomb they are missing on top of their library with every search effect, and choose whether or not they want to draw it if they have a Sphere in play and a Wurm in their library. It's a marginal advantage, but a significant one. If this trick is legal, I intend to play with it.