MTG Keyword Rules — Comprehensive Rules Glossary

To “collect evidence N” means to exile any number of cards from your graveyard with total mana value N or greater.

If a player is given the choice to collect evidence but is unable to exile cards with total mana value N or greater from their graveyard (usually because there aren’t enough cards to do so) they can’t choose to collect evidence.

A spell that has an ability that allows a player to collect evidence as an additional cost to cast it may have another ability that refers to whether evidence was collected. These abilities are linked. See rule 607, “Linked Abilities.”