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Threshold — As long as seven or more cards are in your graveyard, Cephalid Sage has "When Cephalid Sage enters, draw three cards, then discard two cards."
If you don’t have enough cards in your graveyard at the moment Cephalid Sage enters the battlefield, its threshold ability won’t trigger. If the number of cards in your graveyard changes and Cephalid Sage loses the ability once it has triggered, the ability will still resolve.