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Mirage arcane is a powerful illusion spell that:

includes audible, visual, tactile, and olfactory elements

However, the spell description adds:

Creatures with truesight can see through the illusion to the terrain's true form; however, all other elements of the illusion remain, so while the creature is aware of the illusion's presence, the creature can still physically interact with the illusion.

A creature without truesight cannot move through a wall created by Mirage Arcane without destroying the wall. Can a creature with truesight do so?

It seems like the answer hinges on exactly what work the last "can" in the above quote is doing.

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The creature can ignore or interact with the physical parts of the illusion at their choice.

As you say, this really comes down to the meaning of the word "can" here:

the creature can still physically interact with the illusion.

"Can" is not a defined rules word, so it defaults to the idiomatic English meaning of the word: "be able to". Nothing about the word states that they must or must not, just that they are able to. And of course if they are able to do it, they are able to not do it.

Thus, the creature with truesight is able to interact with the illusion, but can choose not to.

Supporting this, Jeremy Crawford has helpfully clarified the meaning of this word in rules context however in this tweet:

In D&D rules, "can" means what it means in English. If you can do something, you're capable of it. You decide whether you do it.

So, can here implies an ability to do something but also a choice.

So, the creature could move through the wall or they could, for example, choose to climb up it (using the appropriate climbing rules of course).

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    \$\begingroup\$ I think you're focusing on the wrong part of the description, and thus misinterpreting the "can" here. I've elaborated in my own answer, but briefly: They're saying you don't lose the ability to interact with it, not that you gain the ability to choose. It's physically there whether you know it's illusory or not. \$\endgroup\$ – Phillip Longman Nov 30 '19 at 22:23
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    \$\begingroup\$ @PhillipLongman I think you make a very compelling argument in your answer. I'll have to think about it and see if I can make any changes to my answer without changing it significantly, but +1 in the meantime for sure. \$\endgroup\$ – Rubiksmoose Dec 2 '19 at 14:21
  • \$\begingroup\$ Can in the sense of able to doesn't always imply the option not to. E.g. a functioning heart can (is able to) pump blood. A person with color sight can see (is able to) color. Neither of those abilities imply a situation where the possessor of the ability has options to turn them off. \$\endgroup\$ – GcL Apr 29 at 2:44
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Respectfully, Rubiksmoose and NathanS are wrong. They are focusing on the wrong word in the sentence, ignoring its most crucial clause, and not thinking about similar spells.

Creatures with truesight can see through the illusion to the terrain's true form; however, all other elements of the illusion remain, so while the creature is aware of the illusion's presence, the creature can still physically interact with the illusion.

The tactile element of the illusion is unchanged, so the purpose of "can still" in this sentence is to make it clear that they don't lose the ability to interact with it. But nothing in the description indicates they gain the ability to ignore its physicality.

Illusion spells in D&D pretty much universally fall into three categories (something more clearly delineated in previous editions):

  • Illusions/figments: Holograms. Sights and sounds that are objectively present but not tangible. They can never hurt you directly.
    Examples: Minor Image, Silent Image, Magic Mouth
  • Phantasms: Hallucinations. Sights and sounds that only exist in the mind. They can hurt you if you believe in them, but the damage is mental.
    Examples: Phantasmal Force, Phantasmal Killer, Weird
  • Shadows: Holodeck holograms. Illusions suffused with shadow-stuff that makes them semi-real. They are objectively present and can potentially hurt you even if you don't believe them, but they'll typically hurt more if you do.
    Examples: Phantom Steed, Creation, Simulacrum, Illusory Dragon

They didn't spell it out in the description, but Mirage Arcane is pretty clearly shadow magic. The effect occupies a physical space, not a mental one, and has physical tangibility.

Truesight doesn't prevent a Simulacrum from casting a spell at you. It doesn't prevent an Illusory Dragon from breathing at you. And it doesn't let you walk through a wall created by Mirage Arcane.

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    \$\begingroup\$ It’s recommended to just answer the question itself rather than to critique/argue with other answers. It’s not necessary, and it makes your answer’s actual point harder to understand because the reader has to go read some other answer first to understand yours—and they might not bother to come back to yours. The best way to get one’s answer considered fairly is to make a positive argument for its point that stands on its own, rather than a negative argument that is entangled with other answers. (Also note that answers here are not “threaded”, and are not usually read in chronological order.) \$\endgroup\$ – SevenSidedDie Dec 1 '19 at 1:04
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    \$\begingroup\$ (That note is motivated by mostly agreeing with this answer and wanting it to be more successful by being more accessible!) \$\endgroup\$ – SevenSidedDie Dec 1 '19 at 1:47
  • \$\begingroup\$ Sorry, I didn't realize that was the norm here. People on StackOverflow questions reference other people's answers all the time. \$\endgroup\$ – Phillip Longman Dec 2 '19 at 1:15
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    \$\begingroup\$ No problem! StackOverflow is… a bit messy. It’s too big and moves too fast for good habits to spread consistently. \$\endgroup\$ – SevenSidedDie Dec 2 '19 at 2:18
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    \$\begingroup\$ I think you make a very compelling argument. I'll have to think about it and see if I can make any changes to my answer without changing it significantly, but +1. \$\endgroup\$ – Rubiksmoose Dec 2 '19 at 14:20
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The creature can walk through the wall

I think you've almost answered your own question there. The part of the quote is:

the creature can still physically interact with the illusion.

which implies that it can also not interact with it and walk right through it. This would make sense given that the creature knows that the illusion isn't there.

If the spell was supposed to force creatures with truesight to be restricted by the illusion, it would probably have been worded to state that more explicitly (given that it has already called out truesight explicitly).

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