Let's say I activate Rootwalla's ability, then it loses it (maybe Humility entered the battlefield), then it gets it back again on the same turn (someone destroyed the humility). Can I activate it again?

Boy that's a tricky one! I actually don't think the rules cover this situation, which is pretty rare for the rules not to do! My instinct was to say "no", but I think the closest answer based on the rules I'm reading is "yes". When a card loses its abilities, those abilities are lost. It's never actually stated what happens to those abilities that brings them back if the effect that removed them goes away. It's obvious that this is what happens, because the game is programmed that way, but there's nothing that actually says abilities that have been lost come back later. By that logic, in order to get them back, the ability must be "gained" due to the object's own rules text. This would make it a NEW Rootwalla ability, and be eligible to being activated again.

I think this actually needs to be submitted to Wizard's though. This is something that the comprehensive rules should be stating.

I disagree with that reasoning. The way continuous effects work is that you start with what's printed on the card and then work your way through the layer system.

With just Rootwalla: No effects, the Rootwalla just has the ability from its printed text.
With Rootwalla and Humility: Rootwalla starts with the ability from its printed text, but this is removed in layer 6 (and there's a layer 7b effect for P/T).
After the Humility leaves the battlefield: No effects, the Rootwalla just has the ability from its printed text.

I would say that it's the same ability each time. In every case, it's the ability generated by its rules text, and that's how it always is for layers 1-5. It's just that sometimes there's an effect in layer 6 that removes it and sometimes there isn't.

So I would reason that it couldn't be activated again.

Okay, because I'm thinking;




So, it has that ability because its rules text gives it that ability (rule 112.1a), but then when it loses the ability the ability is truly LOST (rule 112.10b), even though it still has that rules text on it. Lost abilities can't get applied back to it (rule 112.11). When the Humility continuous effect is lost, the rules text gives the ability back to Rootwalla, meaning that it effectively "gains" the ability. As it has now gained this ability from its rules text, it is technically a new ability, and can be activated again. This is my thought process, but I admit it feels really weird.

112.10b just means that if you cast two Jumps followed by one Canopy Claws on a creature, it won't have flying. And 112.11 is referring specifically to the Archetype of Courage cycle (plus Arcane Lighthouse) and the implication of those cards' "can't have" wording. Neither of those rules is about Humility.

You're right. I was reading "effect can stop an object from having" part to include losing it, which isn't the same. If you cast Jump, followed by Canopy Claws, followed by Jump again, you'd have flying again. Okay, that makes sense. So the text gives it the ability, and the ability is always there but is "lost" in the layer, which, when gone, means it doesn't "gain" the ability, but rather just "has" it again.