
Olivier Rayé-Lalonde RPG Superstar 2015 Top 32 |

The cleric and wizard explicitly state that they can keep some slots open. The arcanist doesn't, so he can't.
There's an exploit that works around this absence of open spell slots: Quick Study. For a full round action and a point from your pool, you can replace a spell prepared with another one from the same level.
Not taking 15 minutes also means that it could, if there is really a need for it, be used during combat.

Dave Justus |

Actually it isn't wizard and cleric. The magic section gives that ability to wizards and divine spellcasters. Of course when it was written, that was all prepared spells.
I think most reasonable people would allow classes such as a Witch and a Magus the same ability, although that isn't explicitly stated.
Arcanists are a somewhat different thing though, since in a way Arcanists aren't preparing a spell that is then expended when cast, they are instead effectively preparing a sorcerer spell slot. In my opinion this is different enough to justify treating it different.
Strictest RAW though it is only Wizards and Divine Spell Casters. (Alchemists have a similar but slightly different mechanic)

wraithstrike |

Preparing Spells in Open Slots: The Magic chapter says a wizard can leave spell slots open and prepare spells in them later. Can any other class do this?
That text was written when wizard was the only class that prepares arcane spells. This option is also available to magus and witch characters (both of which are classes that prepare arcane spells).
It seems the idea was to allow any class that prepares spells to do this.
Also some divine casters are not even prepared casters so the rules have to be read in context to get the correct answer.

Claxon |

The arcanist is a weird hybrid of spontaneous and prepared caster.
While prepared casters can fill spell slots later on, the arcanist doesn't prepare spell slots exactly. He prepares spells known for the day. He then acts as a sorcerer for the rest of the day.
I think there is enough of a difference that the FAQ mentioned above might need to be amended to determine how the arcanist should interact with it.