Logic Knot

Delve (Each card you exile from your graveyard while casting this spell pays for {1}.)
Counter target spell unless its controller pays {X}.

"Such sloppy work. Let's unravel it and try again!"

Time Spiral Remastered

Modern Masters

Delve (You may exile any number of cards from your graveyard as you cast this spell. It costs {1} less to cast for each card exiled this way.)
Counter target spell unless its controller pays {X}.

Future Sight

Delve (You may remove any number of cards in your graveyard from the game as you play this spell. It costs {1} less to play for each card removed this way.)
Counter target spell unless its controller pays {X}.

Because delve isn't an alternative cost, it can be used in conjunction with alternative costs, such as flashback. It can also be used to pay for additional costs that include generic mana.

Delve doesn't change a spell's mana cost or mana value. For example, Treasure Cruise's mana value is 8 even if you exiled three cards to cast it.

To cast Logic Knot, you choose a value for X, determine the total cost, then pay the total cost with some combination of mana and exiling cards.

You can exile cards to pay only for generic mana, and you can't exile more cards than the generic mana requirement of a spell with delve. For example, you can't exile more than seven cards from your graveyard to cast Treasure Cruise unless an effect has increased its cost.