As the title implies, I'd like to learn more about how "are considered" effects interact with each other.
For example, a Hunter gains access to both Druid and Ranger spell lists, which means at level one they have access to both Shillelagh and Lead Blades. To reduce searching, I'll quote the poignant parts of both.
It deals damage as if it were two size categories larger (a Small club or quarterstaff so transmuted deals 1d8 points of damage, a Medium 2d6, and a Large 3d6), +1 for its enhancement bonus.
All melee weapons you are carrying when the spell is cast deal damage as if one size category larger than they actually are.
For the sake of not quoting every effect, I'll simply list the remaining as if/are considered items/abilities which can increase damage of some type of weapon (generally unarmed strike) that I can remember:
Belt of Thunderous Charging, Impact Weapon enchantment, Strong Jaw, Animal Aspect[Gorilla], Brother of the Seal prestige class (7th), & some mythic items/feats.
Another effect that I've questioned is Muleback Cords in conjunction with a Masterwork Bakcpack.
Long story short:
1) Would Muleback Cords stack with a masterwork Backpack, even though both are worded as "treating strength higher?"
2) Do "are considered" effects stack?
-What can be used to define whether an effect stacks with a different, but similar effect, such as source, targeted object(You vs Your items), activating action (Passive vs on charge), or something else?
Effective size increases do not normally stack with each other, but they do stack with actual size increases.
So it would depend on the two specific things.
As an example improved natural attack increased the base damage of your attack so it works with strong jaw. If you had another ability that gave an effective size increase then I don't think it would stack with strongjaw because both would reference the actual damage die.
I don't know that I can give a definitive answer, I think it depends, and should be assessed based on the specific effects.
[I don't think there should be a definitive answer either, I think a hard and fast rule could be harmful]
To break down some of the examples and my reasoning:
Muleback cords affects you magically. MW backpack is as much about balance and packing as it is anything else and is a very different effect to the cords so I can see no reason for them not to stack. (FWIW neither of these items use the phrase 'as if'.)
'as if' doesn't actually change the base damage, so my default position is that two effects of that nature (such as shillelagh and lead blades) don't stack. If however they were very different effects, such as the belt of thunderous charging and lead blades, then they should stack.
Something like Animal Aspect (gorilla) actually increases the damage die, and does so without the 'as if' proviso, that would likely stack with similar effects and stack with an 'as if' effect.
Lead Blades and Shillelagh aren't really a 'stacking' question. They both do exactly what they do.
Shillelagh makes a medium club do 2d6 damage.
Lead Blades, (cast on a medium club) makes the weapon do 1d8.
Neither spell would care if the other spell was cast on the item already or not. The only real question is if lead blades reduces the damage of Shillelagh or not (I would say not due to 'increases' in the text).
Shillelagh doesn't make any reference to the original size, so I see no reason why it wouldn't modify a weapon with Lead Blades.
No, because they are mechanically the same thing.
In regards to the Shillelagh spell, it has the same language as Lead Blades, it increases X as if it were Y/Z size(s) larger. Things like that don't stack.
Those effects would, however, stack with a spell like Enlarge Person, as that spell increases your actual size, not your effective size damage dice as Lead Blades/Shillelagh do.
It's the same concept with Muleback Cords and the Masterwork Backpack. Because they both treat your score as X/Y higher for determining carrying capacity, they won't stack with themselves because they are the same effect with the same language. A Masterwork Backpack would stack with a Belt of Giant's Strength, though, since one increases your actual Strength score, and the other just treats your Strength score as 1 higher for carrying capacity.
I see the general consensus is no, especially for Shillelagh + Lead Blades.
Lead Blades and Shillelagh aren't really a 'stacking' question. They both do exactly what they do.
Shillelagh makes a medium club do 2d6 damage.
Lead Blades, (cast on a medium club) makes the weapon do 1d8.
Neither spell would care if the other spell was cast on the item already or not. The only real question is if lead blades reduces the damage of Shillelagh or not (I would say not due to 'increases' in the text).
That is a terrible precedent to set. If every ability had their own table for scaling, then none of them should use the phrase "size category" to describe the effective increment in damage. It becomes impossible to gauge what a size category actually means if two skills have the same wording and completely different outcomes.
On a somewhat related note, I'm considering making a universal size table for manufactured weapons and unarmed strikes to help with understanding what a character's final damage actually is. I'd prefer to make each a singular line of progression, but would like to gauge the value of a table using multiple dimensions.
In the case of multi-dimensional tables, I'd need to know whether to apply the damage progression of the table shown in Improved Natural Attack first or should Strong Jaw be the one to applied first.
In regards to the Shillelagh spell, it has the same language as Lead Blades, it increases X as if it were Y/Z size(s) larger.
Not in the version quoted above, it doesn't.