On arcanists using the archmage's Arcane Surge mythic path ability

[Ravingdork]

Arcane Surge:

Arcane Surge (Su) As a swift action, you can expend one use of mythic power to cast any one arcane spell without expending a prepared spell or spell slot. If you prepare spells, this spell must be one you prepared today (even if you have already cast it); if you’re a spontaneous caster, this spell must be one of your spells known. If the spell requires a saving throw, any non-mythic creatures affected by the spell roll twice and take the lower result. If you must attempt a caster level check for the spell to overcome a creature’s spell resistance, you can roll your caster level check twice (adding your tier to each) and take the higher result. You can’t add a metamagic feat to a spell you cast using this ability.

How does this ability work when used by an arcanist? Can he cast any spell in his spellbook (that is, that he knows), or is he limited to those he has currently prepared?

[Eltacolibre]

Most arcanist would just pick Wild Arcana to avoid any ambiguity. There isn't an official answer to the use of arcane surge as an arcanist, but if I were to make a judgement call as a DM:

When you pick arcane surge as an arcanist, choose if you want to be counted as a prepared spellcaster or one with spells known.

[kestral287]

An Arcanist prepares spells. Their casting methodology is very different, but "An arcanist must prepare her spells ahead of time" seems pretty clear to me and is straight out of the RAW. Using Arcane Surge, the Arcanist has to pick one of the spells she's prepared today.

[Avoron]

"If you prepare spells, this spell must be one you prepared today (even if you have already cast it); if you’re a spontaneous caster, this spell must be one of your spells known."

Those two clauses are not mutually exclusive.
An arcanist definitely prepares spells, so the spell must be one they prepared on that day.

It is debatable whether an arcanist qualifies as a spontaneous caster in this sense, as one of the closest things we have to a definition of the term is
"Most spellcasters prepare spells in advance-whether from a spellbook or through prayers-while some cast spells spontaneously without preparation."

But regardless, all that would mean is that the spell also must be one of their spells known.

Whichever way you look at it, it has to be a spell they prepared.

[Ravingdork]

It seems to me that arcanists are both prepared AND spontaneous casters. They prepare their spells, and then they spontaneously cast them.

[Avoron]

That's probably the case. But, like I said, the quote doesn't exclude both from being the case.

If you are a spontaneous caster who prepares spells, this spell must be one of your spells known and this spell must be one you prepared today.

Basically, the quote says, "If X, they A; if Y, then B."
So, "If X and Y, then A and B."
At least, that's my interpretation.

[kestral287]

Certainly. But as Avoron notes, nothing in the two is inherently exclusive. If an Arcanist is both, then she would still have to meet the criteria of having prepared the spell that day, because "both" includes by definition "prepared caster", so she has to follow those rules. It just so happens that she also has to follow the spontaneous rules, but she'll meet them by default.

If the Arcanist was a prepared caster or a spontaneous caster (i.e., he could either prepare spells or spontaneously cast them) then that would be a different matter, but such is evidentially not the case.

But "both" means we have an and situation, and this in turn means she fits into both of the options presented here-- but not in such a way as she gets to pick, but in such a way as both apply.

There's no arcane 'or' caster that I can name off-hand, but for an example of what I mean here: the Druid can cast normal Druid spells as prepared spells or spontaneously cast Summon Nature's Ally spells. Thus one could make a case for a Druid being able to use the Hierophant's version of this ability to cast a Summon Nature's Ally even if it wasn't prepared that day (she would have to have it as a spell known, but that's not a problem for a Druid).