If there were a mechanism whereby a wizard reliably could cast a spell he does not "know" but still has access to (such as by having the grimoire it is written in at hand) I could see this scheme working.
But having the spells known limit to be a hard absolute limit means that plundering lost libraries and enemy mages for their spellbooks is too weak, without any oomph. More importantly, an comparison to literature (Appendix N or otherwise) must always be made loosely. Writers only add in stuff they know will be relevant. Role-playing games do not have that luxury.
The idea of a Wizard being able to cast many and varied spells, given enough preparation, is a good one, and I would have preferred it if DCC did not go counter to it. There are many stories that simply cannot be told if the game rules prohibit a party from casting a spell important to the plot. Which brings me back to how the rulebook limit could have worked, if there was an allowance for a "safety valve", a way to cast spells from other sources than the brain of the caster (where the Spells Known limit is imposed).
I do recognize the counter argument "wizards lose their individuality if they can all learn all spells, or at least the best ones". I just think I won't play so many DCC campaigns that this will ever become an issue. I also understand the analogy with warriors. If fate gives you a character with low Strength you can't easily fix that by just studying for a week or three. But if you can switch out your crap spell for a good one then that is just what a Wizard can do. The most immediate counterargument to this is simply "flexibility is precisely what a Wizard is supposed to be about". The whole point of using your brain instead of your brawn or your belief is that you are only constrained by your wits and your imagination.
A role-playing game is storytelling but also a game. And from the game POV, having additional spells be only candidates for very limited slots is a very harsh restriction. As explained above, the intention is that a Wizard might well have found five spells, but each time he levels up only one can be selected. For starters, this basically means no Wizard will ever learn more first level spells once they reach 3rd level (since doing so would mean learning one less spell of the highest level you can cast). That's an awfully high opportunity cost.
Take all of this into account, and my evaluation is that a simple (and hard) limit is removing more than it adds, if we view the game rules as a necessary evil to facilitate the telling of stories. For many reasons, I will therefore lift the hard limits as detailed above.
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When it came up last night I was unprepared for the lack of explanation (assuming that "only 18 spells full stop" does require an explanation if you have ever played any other iteration of D&D).
Having to make an on-the-spot decision, I ruled Spells Known is in actual fact
Spells Memorized (this was mentioned upthread though not using those terms).
Looking now at how the rulebook proposes new spells I am amazed how compatible everything remains, simply by this "minor" (read not minor at all) tweak.
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Specifically: when a mage finds a spell in a book or otherwise, defeats its guardians and breaks any encryption (and so on
) make an
Understand Spell check:
The check consists of 1d20 plus his caster level plus his Intelligence modifier. The DC is 10 + spell level. (The specifics are unchanged from the rules). If he passes, he
understands the spell. This means he's able to reproduce it, write it down, cast it and so on. Roll for Mercurial to individualize the spell. If he fails, he cannot understand or use the spell. If the Judge is lenient, he can try again after gaining a level or after a year (etc).
This leaves a Wizard PC with a spell book containing more spells than he can memorize at any given time. (The number of spells you can hold in memory at any given time is the number Spells Known in the rulebook)
To memorize a new spell (from a source you understand) you spend the requisite time (given in the rulebook as one week per spell level). The Judge may still require additional effort, such as "a price paid" or similar. Otherwise this process is automatic.
Of course, in any campaign where you only have a few weeks between adventures this means you can't swap your entire "spell loadout" between adventures. If you spellburned yourself out completely last adventure, maybe there's not even time for one such swap!