You can think of Spells like Ways or Events. They allow you to play a potion during your action phase (doesn't use up an action) or buy phase (doesn't use up a buy) to gain the effect on the Spell instead of using it as a typical treasure this turn. They would be added to games that already have potions, so that potions would always have at least 2 uses: to buy at least 1 card, and to use as a spell.
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