Can Defy be used on Sky Splitter when its cost has been reduced to 0?

Ruling: No, Defy cannot be used on Sky Splitter when its cost has been reduced to 0. Effects that check a card's cost only look at the card's printed cost, ignoring any reductions.

Nuances:

• Cost reductions do not affect abilities that reference a card's cost
• The printed cost is what matters for card effects that check cost values