Ancestral Vision’s mana cost is an enigma wrapped in a mystery. Actually, Ancestral Vision doesn’t have a mana cost at all. It’s not 0 — cards with a mana cost of 0 exist, and they actually have the number 0 in their upper corner, such as Lotus Petal. Rather, just like a land, Vision’s mana cost simply doesn’t exist. As a result, that mana cost can never be paid, no matter how hard you try.
In spite of not really existing, Vision’s mana cost can still be used to set the value for other effects. Whenever another effect asks Ancestral Vision what its mana cost is (as opposed to its converted mana cost, which would indeed be 0), Ancestral Vision shrugs and says “my cost doesn’t exist.” This is a perfectly valid answer, so the game accepts it as-is, rather than interpreting it as zero. As a result, if you cast Snapcaster Mage and target the Ancestral Vision in your graveyard, Ancestral Vision will gain flashback — but the flashback cost will also be nonexistent, and therefore unpayable. So you can’t flashback Ancestral Vision.
So what’s a blue mage to do? Fortunately, you can still cast Ancestral Vision if you find a way to do so without paying its mana cost. In fact, this is how suspend normally works — part of the rules text for suspend says “When the last time counter is removed, cast this card without paying its mana cost.” “Without paying its mana cost” is an alternative cost, and it’s perfectly legal to cast Ancestral Vision by paying an alternative cost. It just so happens that the alternative cost in this case is the delightful cost of free.
Another common way of casting Ancestral Vision is via cascade. Even though its mana cost is nonexistent, its converted mana cost is still definable. Because the mana cost has no mana symbols in it (or anything at all, really), the converted mana cost is 0. As a result, you can cast Vision off of any card with cascade, such as Shardless Agent!
Today’s Rules Tip written by Paul Baranay