I was ruminating on the subject of Vancian magic and how it relates to Dungeons & Dragons* spells are memorized; they are forgotten when cast; they may be transcribed from a scroll to one's spellbook or cast directly from a scroll, but vanish from the scroll when so used when it struck me: in a world where a spell disappears if it is cast from the page instead of memorized, spells would become scarce quickly if they ever fell into the hands of those who had not been properly trained in the magical arts. A wise spellcaster would know never to cast a spell directly from a spellbook or a scroll if that spell were not already recorded elsewhere. Magic is the greatest treasure to a wizard, to be preserved at any cost, so there will be no casting from scrolls unless the wizard created the scroll for that purpose or found a scroll containing a redundant spell. The only ones foolish enough to cast spells directly from their source without a copy in safe storage would be disobedient apprentices and non-spellcasters. In D&D, that means thieves, although logically it would mean anyone. (The nonsense of thieves having a special affinity for magic owes more to crafting a Grey Mouser class than creating a credible thief class, but the implications remain the same regardless.) Just imagine the chaos of common rabble using spells indiscriminately, rejoicing in their fleeting power, unaware or unconcerned that this may be the last time a given spell is ever cast, for it may be the last written record of that spell. Perhaps this is the reason so much arcane knowledge has been lost (presuming there was an earlier era of higher magic). The written spell, evaporating from the page as it is incanted, disappearing forever from potentiality, living on only in tales. It is a melancholy fate for magic, like the loss of ancient literature or music. Every age is a post-apocalyptic age of one sort or another.
* OD&D, AD&D, and certain other editions/versions.