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Recently, it occurred to me that besides flavor stuff like where the magic comes from and what spells they can cast, I can't really think of anything that's different between divine casters, arcane casters, and psychic casters.

As seen in this spreadsheet comparing the spellcasting classes, many caster classes have archetypes that switch casting stats and/or casting types to other kinds, which seems to imply that something changes other than flavor, but for the life of me, I can't see it. Changing to spontaneous from prepared and vice versa as well as changing casting stats is going to have obvious mechanical effects, but what about changing sources?

What mechanical effects does changing a casting source (i.e. divine, arcane, psychic etc) have on the caster itself?

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Each one has its own advantages, drawbacks, and restrictions

Arcane Casting:

Arcane casting is the only form of casting with the significant drawback of Arcane Spell Failure Chance. Most pieces of armor, unless made of special materials such as Mithril, have what is know as Arcane Spell Failure Chance, represented by a % (usually divisible by 5). The heavier/more protective the armor, the higher % chance for failure there is. % chance from a held shield is added to the % chance from any worn armor.

What this does % chance does is every time a parson casts an arcane spell that has a somatic component (that is, you need to move your hands in a particular motion as part of the casting), they roll a d% and determine of the armor interfered with the spell to much to be able to cast.

The primary advantage of arcane spells is that the spell list has, in general, the most powerful and highest utility spells.


Divine Casting

Divine spells, on the other hand, are not subject to a chance to fail when cast, even when the caster is in the heaviest armor imaginable. Most divine casters also have restrictions on what kind of spells they can cast based on what deity they worship. The only real drawback to Divine casting is that the spell selection for Divine casters is generally worse that the options for the equivalent arcane/psychic caster.

Divine spells typically aren't quite as good as an Arcane counterpart spell (compare Haste and Blessing of Fervor), but then again, you can cast them while strolling around in full plate and carrying a tower shield, and not have to worry about if they'll actually go off.


Psychic Casting

Psychic casting is the oddball of the three. You see, psychic casters don't perform somatic or verbal components on their spells like Arcane of Divine casters, and, since casting a psychic spell is a purely mental action, they can cast even while grappled/bound/paralyzed/etc. Instead of verbal or somatic components, they have emotion and thought components. Additionally, they can substitute items of significant importance (of equal or greater value) for material components for a spell. The example given in the books is using a wedding ring (worth 5K GP or more) in place of the diamond dust to cast Raise Dead on a spouse. The last advantage of psychic spells is that some of them can be undercast. That is to say, if you have (for example) Mind Thrust IV as one of your spells known, you are considered to know (for anything that requires it as a pre-requisite), and are able to cast, Mind Thrust I-III, using the appropriate spell slot. Meanwhile, an Arcane/Divine caster must have each known and/or prepared separately.

The main strength of Psychic casting is also it's main weakness: how the components differ. Any time someone is under any sort of non-harmless effect with the emotion or fear descriptor (such as a Red Dragons Fear Aura, or the Demoralize effect of the Intimidate skill) they cannot fulfill the emotion component of a spell. A spell with a thought component has a concentration DC of 10 higher than normal, unless the caster takes a move action prior to casting the spell to "center themselves".

The Psychic spell list is a compilation of what spells would be traditionally considered either Arcane or Divine "only" (in quotations because they are always ways to get a spell you normally wouldn't), but it doesn't have most of the signature powerhouse spells, like Wish and Miracle, and there are a (very small) number of spells that are Psychic only, like Mind Thrust I-IV and Instigate Psychic Duel.

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  • \$\begingroup\$ I seem to recall morale effects, even morale bonuses, interfering with psychic spells. I specifically remember inspire courage being a problem, absurdly. Is that me misremembering, or did they errata that? \$\endgroup\$
    – KRyan
    Apr 9 '20 at 19:49
  • \$\begingroup\$ @KRyan You're probably mis-remembering, or possibly remembering 3/.5e stuff as Pathfinder. The Pathfinder psychic spell rules explicitly exception harmless emotion effects, and most morale bonuses would be considered harmless effects, including Inspire Courage. \$\endgroup\$ Apr 9 '20 at 20:19
  • \$\begingroup\$ D&D 3.5e had nothing remotely similar to thought and emotion components. \$\endgroup\$
    – KRyan
    Apr 9 '20 at 20:51
  • \$\begingroup\$ Well, I've no idea then. \$\endgroup\$ Apr 9 '20 at 21:01
  • \$\begingroup\$ Your example about undercasting Beast Shape IV is incorrect. The spells have to call out that they can be undercast a la ego whip or Mind Thrust. Beast Shape, like Contact Entity and Summon Monster, lacks this language and requires a psychic caster to take it for that level. Mind Thrust (not Spike) also goes to VI. \$\endgroup\$
    – willuwontu
    Apr 11 '20 at 19:32
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Spellcasting type (e.g. arcane, divine, psychic) has many mechanical implications. Some are built into the spellcsating rules, such as spell failure for arcane casting and psychic spell components. Other mechanical implications are due to interactions with an external game mechanic that refers to the spellcasting type. So it is not simply "flavor". Below are some examples.

Arcane spell failure

Wearing armor or a shield can impose arcane spell failure chance, a percentile chance that the spell has no effect, for arcane spells with somatic components. Certain feats and features can bypass this (such as a bard in light armor) but those are exceptions to the rule. Arcane spell failure chance does not apply to divine or psychic spellcasting.

Psychic spell components & concentration

Whereas arcane and divine spells typically have verbal and somatic components, psychic spellcasting requires emotion and thought components. These use their own separate rules and may have different calculations for concentration check DC. Metamagic feats like Silent Spell and Still Spell don't apply to psychic spellcasting.

Spell scroll restrictions

As addressed in this question about arcane vs divine scrolls, casting a spell from a scroll requires that the caster's type matches the scroll's type. Whether the scroll itself is arcane, divine, or psychic is determined by the scroll's creator, who may be different than the character reading the scroll.

Prerequisites

Certain feats, items, or other features require some form of spellcasting of a given type. For example, the dragon disciple prestige class requires 1st level spontaneous arcane spells, so an oracle (spontaneous divine caster) or mesmerist (spontaneous psychic caster) would not qualify.

Targeting

Certain spells and abilities only target spellcasters of a certain type. Others have varying effects depending on how another creature casts spells. For example, if the target of a feeblemind spell can cast arcane spells, then they take an extra penalty on their saving throw against the spell. Similarly, some effects protect against spellcasting of a certain type (such as divine defiance boosting saving throws against divine spell effects).

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  • \$\begingroup\$ It should be noted that thought and emotion components can be really big problems. It’s much easier to prevent someone from performing those than it is with somatic and verbal, and worse yet, some should-be-beneficial effects block them. Much easier to be shut down as a psychic caster. Also, alignment issues for divine spending deserves a place here—strictly speaking, that’s the property of particular “aligned spells” class features, and not automatically a part of divine spellcasting, but most GMs, I imagine, will enforce it even if something neglects to mention it. \$\endgroup\$
    – KRyan
    Apr 9 '20 at 18:53
  • \$\begingroup\$ You should also note the different ways they prepare their spells. \$\endgroup\$
    – willuwontu
    Apr 9 '20 at 19:14
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    \$\begingroup\$ @williamporter Spell preparation seems to vary by class. Some divine casters prepare spells differently than others. For example, the living grimoire inquisitor prepares spells from a book, unlike a warpriest or druid who can choose from their class spell list. \$\endgroup\$
    – MikeQ
    Apr 9 '20 at 19:20
  • \$\begingroup\$ @KRyan is right. A single Intimidate check to demoralize could prevent a psychic spellcaster from casting any of their spells with an Emotion component for several turns, and a spellcaster who can't cast spells is very weak. \$\endgroup\$ Apr 9 '20 at 19:47

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