MTG Keyword Rules — Comprehensive Rules Glossary

“Discover N” means “Exile cards from the top of your library until you exile a nonland card with mana value N or less. You may cast that card without paying its mana cost if the resulting spell’s mana value is less than or equal to N. If you don’t cast it, put that card into your hand. Put the remaining exiled cards on the bottom of your library in a random order.”

A player has “discovered” after the process described in 701.57a is complete, even if some or all of those actions were impossible.

If the final card exiled during the process described in rule 701.57a has mana value N or less, it is the “discovered card,” regardless of whether it was cast or put into a player’s hand.