Fun with Gideon and layers

Gideon Jura is one of the most interesting planeswalker cards because he has a rather particular ability: He can (temporarily) become a creature. (It's certainly possible to make any planeswalker permanent into a creature via trickery, but Gideon doesn't need the trickery because it's directly one of his abilities.) This allows very interesting interactions and situations both in the game mechanical and the flavor sense.

What exactly happens when Gideon's third ability is used? The question seems rather self-evident (he "becomes a 6/6 Human Soldier creature that's still a planeswalker"), but it's in fact a bit of a complicated issue because it has to do with the concept of layers. In the vast majority of cases one doesn't need to care about this at all, as a straightforward approach is enough to get it right. However, let's examine what's happening a bit in more detail.

An interesting question is: What's Gideon's entire type line and power/toughness after the third ability has been used? While there is no existing template for a card that's both a planeswalker and a creature, it would nevertheless be something along the lines of this:

In other words, his entire type line is:

Planeswalker Creature — Gideon Human Soldier

He is both a planeswalker, of subtype Gideon, and a creature, of subtypes Human Soldier. As a creature, his power and toughness are 6/6.

Ok, so far there's nothing special about this and, as said, this suffices for the vast majority of cases. However, there are situations where this becomes a lot more complicated.

There exists a wonderful card named Mirrorweave, which is an instant that says: "Each other creature becomes a copy of target nonlegendary creature until end of turn." This card is wonderful because it allows making any creature a copy of any other creature, resulting in really interesting shenanigans with respect to the layering system of the game.

Let's assume that in addition to Gideon Jura (whose third ability has been used), there's for example a Grizzly Bears on the battlefield. It looks like this:

Now someone casts Mirrorweave targeting Grizzly Bears. What exactly happens to Gideon?

A good majority of players would hastily say that Gideon just becomes a perfect copy of Grizzly Bears, and that's it. In other words, he becomes a green 2/2 Bear creature (with converted mana cost 2, and named "Grizzly Bears".) However, that's not correct. The correct answer is that he becomes this:

In other words, he becomes a green 6/6 Bear Human Soldier creature (with CMC 2, named "Grizzly Bears", without abilities, and that's not a planeswalker.) But why?

The answer is a bit complicated, and has to do with layers.

Mirrorweave's copy effect is applied on layer 1, ie. before anything else. This makes the Gideon Jura permanent a perfect copy of the Grizzly Bears card (all of the copyable values are copied.) None of the contents of the Gideon Jura card are retained because the copying effect replaces them all (which is why he stops being a planeswalker, alongside everything else.)

However, the story doesn't end there. Gideon's third ability is still in effect (because once it's activated it applies to the permanent until end of turn, no matter what else is applied to the permanent in addition.) Said effect says that he "becomes a 6/6 Human Soldier creature."

The creature subtype setting effect is applied on layer 4, making him a Human Soldier, and the power/toughness setting effect is applied on layer 7b, making him 6/6. In other words, these two effects apply on top of Mirrorweave's copying effect.

There's still one small catch. Namely: Why is he still a Bear, if his own ability is making him a Human Soldier?

This is a very relevant question because normally if an effect sets the subtype of a permanent, it replaces any subtypes of the same category that permanent may have had. For example, if Gideon were to now be enchanted with Lignify, which says "enchanted creature is a 0/4 Treefolk with no abilities", his type would become "Creature - Treefolk". All of his existing subtypes would be replaced by Treefolk. (The type setting effect of Lignify trumps Gideon's because it has a later timestamp.)

Therefore the question is: If Gideon's third ability says that he "becomes a 6/6 Human Soldier creature", then why does he retain the Bear subtype?

The reason for this is that the effect in question is, actually, "becomes a 6/6 Human Soldier creature that's still a planeswalker." That last part is crucial. As unintuitive as it may sound (and it indeed is quite unintuitive), "is still a planeswalker" means that he retains any types he might have in addition to gaining "Human Soldier creature".

There's a section in the comprehensive rules that states, basically, that "is still a X" is a synonym for "in addition to its other types" (completely regardless of what that X might be.) Therefore whenever you read "is still a X" in a card, it's completely equivalent to it reading "in addition to its other types" instead. For this reason Gideon retains the Bear subtype in this situation. (Perhaps a bit confusingly "is still a planeswalker" does not add the planeswalker type. It's not an effect that adds anything.)

This peculiarity, however, allows some rather interesting situations flavorwise. For example, let's assume that instead of Grizzly Bears, the other creature had been a Dryad Arbor:

If Mirrorweave were targeting that, what would Gideon Jura look like? This:

In other words, his type line becomes:

Land Creature — Forest Dryad Human Soldier

Because "is still a planeswalker" means that he retains all of the types that Mirrorweave copied, he is now a Forest land in addition to being a Dryad Human Soldier creature. Yes, he can tap for one green mana (but has no other abilities.)

Also, curiously, since he's not a planeswalker at this point, you can cast another Gideon Jura without the planeswalker uniqueness rule kicking in, so you can have two Gideon Jura cards on the battlefield at the same time (but only until end of turn, of course.)