How do you balance Keen Mind with Wizards? By reading what Keen Mind does, it would allow for a Wizard to just see a spell being used and then can use it themselves. There is MUCH MORE to casting a spell beyond a gesture and a sound. The mind of the mage has memorized elaborate equations and glyphs they weave in their mind’s eye as they cast it. Just seeing a spell be cast wouldn’t enable you to.
— Matthew Mercer (@matthewmercer) December 22, 2018
So can wizards use the keen mind feat to effectively bot need their spell books for 30 days?
No — the metaphysics behind how spells are prepared and “known” are beyond the context of mundane memory. The ability to cast a spell needs to be set mystically into the mind/soul of the caster — deeper than what the words/images in a spellbook mundanely convey. When a spell is switched out for whatever reason, the metaphysics behind how the caster is able to cast the spell are literally gone. To put it another way: the simple words, tones, gestures, etc, that a wizard does when they cast a spell are the tail-end, most superficial (though still necessary) aspect of casting a spell, similar to tapping a few commands on your phone’s app or speaking into it; the metaphysical underpinning of how the spell actually manifests through the wizard, however, is predicated on the megs/gigs of programming inside your “phone/computer” (the wizard’s soul/mind). The wizard can only have “x” number of spells (“apps”, as it were) at any time. When they don’t have it prepared, they can very well speak the words and gestures to cast, say, Magic Missile just as they might otherwise normally do… but nothing happens.
This is why wizards need their spell books as a foundational aspect to aid in opening their minds/souls to “downloading” the essence of the spell from The Weave (something that occurs for other classes via deities for clerics, patrons for warlocks, innate-but-limited ability for sorcerers, etc.). But the mundane words/imagery notable from the book, alone, aren’t enough to do this — it takes the actual act of preparing the spell to have it all come together.
Apparently won’t allow links to SA tweets from Crawford, but he noted Keen Mind not changing how wizards prepare/recall their spells on November 12th, 2015, if you look up his tweets for that day.