Vancian Magic

AGNES: Tilly, shoot them with a magic missile.

TILLY: I can’t. 

AGNES: What do you mean you can’t?

TILLY: I forgot the spell.

AGNES: What do you mean you forgot the spell?

TILLY: It’s a thing. It’s not going to help us. 

RHIALTO THE MARVELLOUS, book IV of Jack Vance's influential fantasy series DYING EARTH.

RHIALTO THE MARVELLOUS, book IV of Jack Vance’s influential fantasy series DYING EARTH.

Click the above image of RHIALTO THE MARVELLOUS to read an informative blog post from The Evil GM on Jack Vance’s influence on D&D’s magic system (Vancian magic.) The basic theory behind this is that a wizard’s spells are so complex (s)he can only keep them in his/her memory until (s)he uses them, after which the spells are completely forgotten and must be re-learned. It is, indeed, a thing.

Also, the song “The Dying Earth” by the band “Wizard Eye.” Also a thing.