Magic Theory is a topic I have long loved to discuss. From how well theories of tempo and card advantage can adapt to a format as broken as Vintage to whether or not Pulse of the Fields can constitute card advantage, or if it's exclusively the province of tempo. Recently, I was greatly dismayed to find the following post in the forums.
Card Advantage does not count your opponents stuff. EVEN if you make him discard 6 cards from his hand, that does not count toward your CA. Card advantage implies that YOU have gained more cards than you played over the course of the game. Board advantage implies that you have gained more permanents in play than your opponent. Playing a wrath effect to get rid of his stuff doesn't gain you CA. It gains you board advantage. BIG DIFFERENCE...
Ouch. The quote above literally caused me to wince in physical pain. It's not so much that the definition is incorrect as that the definition is both impractical and incomplete. What follows is a bare bones skeleton of the most fundementally sound and relevant theory in all of modern Magic.
What is Card Advantage
Over the years, there have been several different definitions of card advantage. I won't offer you one, because merely defining the term can be (and has infact been) the subject of a very lengthy article itself. I will leave you to come up with a working definition that suits your needs. The person above decided to limit card advantage to only being the ratio between cards drawn and cards played. The simplest definition I have found to be functional is: The number of cards going into each players hand versus the number of cards going into each players graveyard. This definition still gets a few things wrong, but they are easy to spot (Call of the Herd and Mental Note being obvious "mistakes"), and easy to correct. An even more complete definiton will have to take into account how relevant the cards in question are. Fog for example, is pretty useless against a deck with no creatures.
The mandatory first example of card advantage is always "I cast Ancestral Recall on myself". For those of us who don't play Vintage, we can pretend that we're casting Petals of Insight, to keep it t2 legal. Regardless, we cast one spell from hand to put three cards into hand. It's a simple case of 3-1 card advantage and even the poor farmer who's trying to milk the chickens can understand it.
What else can generate 3-1 card advantage? What if we lead off the game with Cabal Therapy naming Counterspell? Our opponent reveals a hand of 3 Counterspells and discards them all. I've clearly gained some sort of 3-1 advantage, yes? The first poster says no. I didn't draw any extra cards, and spent a card from my hand. According to his definition, my play was actually anti-card advantage. That's why I think his definition is totally useless. A more rational person would correctly conclude that I lost 1 card, and my opponent lost 3 cards. I've just gained the same NET advantage as casting Ancestral Recall. (author's note: According to the personal definition I use, this play is often even BETTER than casting Ancestral Recall, for reasons that will be explained below)
Another example is my opponent has 3 creatures, and I cast Wrath of God. We still trade 3-1, just like the Cabal Therapy example. Yup, that's card advantage too. Right about this point, my first poster is crying foul. "Wrath of God doesn't create card advantage. It's a TEMPO advantage you fool!" First sentence wrong, second sentence right. If you're already familiar with tempo, the tempo advantage is obvious. If you aren't, then you'll have to wait for my tempo article ;).
Thus far, both definitions I provided will produce the same results. So where does the difference between the two of them lie? Welcome Virtual Card Advantage. When card advantage was first introduced, theory made it up to the examples above, and was called "done". Everyone would live happily every after with a nice little theory, untill some wise guy realized the Wrath of God example was incomplete.
What happens if none of the creatures had flying and there was a Moat in play? Now casting Wrath doesn't look so hot. Those creatures were literally about as useful to my opponent as the Ace of Spades, and since we're not playing poker; that's pretty damn useless. People quickly began to call useless cards "blanked". A completely blank card is totally worthless for the rest of the game. If I have a Damping Matrix and you don't have artifact removal, then I've just permanently blanked that Sword of Fire and Ice in your hand. If all you need to do is draw your Viridian Shaman for my Damping Matrix then the Sword is still blanked, but only temporarily.
Land Destruction is a familiar deck that makes a living by temporary blanking. A turn 2 Stone Rain can blank most of the stuff in your deck by making it impossibe for you to cast it. A Tooth and Nail, looks pretty foolish on only 2 lands in play. Sure eventually you'll cast it, but the LD player is betting he can kill you before that happens. While LD is generally considered a "tempo" deck, it certainly makes sense to think of it in terms of temporary blanking and virtual card advantage.
What makes virtual card advantage so difficult (and so much more useful than conventional card advantage) is that it says not all cards are considered equal. If I'm playing a red deck with Browbeat, drawing Firebolt, Magma Jet, and Volcanic Hammer is different than drawing three Mountains. Correctly assessing how relevant a card is becomes of critical importance. For example, if I want to kicker an Urza's Rage but lack the mana, those mountains are more useful than the three burn spells! A bit counterintuitive isn't it?
Why do I Care?
So what's the big deal anyway? The purpose of Card Advantage is to help your decision making and deck building process by giving you a tool to analyze specific cards, or plays. If you've got a Wrath of God in hand you obviously don't want to play out all your creatures first and wrath second. On the other hand, if you play an Exalted Angel, your opponent will have to play even MORE creatures to race that newfound life gain. After he over-commits to the board, you will happily cast Wrath of God, trading not 3-1, but 7-2. As an 8/8 with Trample, a Force of Nature is one of the most powerful creatures in green today. However, before you throw him in your deck, remember that he's blank until you get , and how likely is it you can get that in time for him to still be relevant?
That's all you need to know to talk card advantage with the best of them in the forums. For a more complete understanding of the more practical side of card advantage, I invite you to do more reading on virtual card advantage and lineup theory, role reversals and inevitability, and even the all encompassing umbrella of "advantage theory" in general. Who knows, if this article gets positive reviews, I might even be back to discuss them myself at some point. Until then, may you never count cards when playing blackjack, and always count them correctly when playing Magic.