Spies of the Mind

What enables the wise sovereign and the good general to strike and conquer, and achieve things beyond the reach of ordinary men, is foreknowledge. This foreknowledge cannot be elicited from spirits; it cannot be obtained inductively from experience, nor by any deductive calculation . . . the dispositions of the enemy are ascertainable through spies and spies alone.
-Sun Tzu, The Art of War

"Sun Tzu held spies in the highest regard, saying that they should be rewarded more liberally than any other relation in war because the foreknowledge they can provide is more valuable than any other commodity in war. When one knows where the enemy will strike, one doesn't need to spread his forces thin protecting a dozen possible targets. When one knows when the enemy is unprepared, one can strike and be assured of victory. When one knows the habits of the enemy general, those habits can be turned against him. The use of spies to gain foreknowledge is like being able to see into the future."

I see only one move ahead, but it is always the correct one.
-Jose Raoul Capablanca,
3rd World Chess Champion


In competitive games, there is little more valuable than knowing the mind of the opponent, which the Japanese call "yomi." All the complicated decisions in game theory go away if you know exactly what the opponent will do next. Sun Tzu says that reading minds is for the spirit world, and on that I cannot comment, but I have witnessed firsthand the ability of some players to "achieve things beyond the reach of ordinary men" through eerily powerful yomi. Perhaps these players are simply adept at "studying the details of the enemy," but it seems to go far beyond that in some. There is one player who I would even say has a supernatural ability to spy on the minds of others, knowing which moves they will next make-if it weren't such an absurd thing to say. But believe me, those who have witnessed Japan's fighting game player Daigo Umehara do speak of these things in hushed tones, fancying that they might be true.

As a side note, I would even argue that the "strategic depth" of a game should be defined almost entirely on its ability to support and reward yomi. For a silly example, consider tic-tac-toe. There are only nine opening moves, and only three of them are functionally different. Even if through some witchcraft you know the move the opponent will make next, it doesn't really matter. The game is so constrained that the opponent is forced to make certain moves, so the novice player along with the master of divination will be on basically the same footing. There is no room to develop "tendencies" or a certain "personality" or style of play in tic-tac-toe. There is only a simple algorithm at work and no room for yomi at all.

Yomi Layers

Any decent competitive game needs to allow you to counter the opponent if you know what he will do. What happens, though, when your enemy knows that you know what he will do? He needs a way to counter you. He's said to be on another level than you, or another "yomi layer." You knew what he would do (yomi), but he knew that you knew (yomi layer 2). What happens when you know that he knows that you know what he will do (yomi layer 3)? You'll need a way to counter his counter. And what happens when he knows that you know . . .

I'll nip that in the bud: there need only be support up to yomi layer 3, as yomi layer 4 loops back around to layer 0. Let's say I have a move (we'll call it "m") that's really, really good. I want to do it all the time. (Here's where the inequality of risk/reward comes in. If all my moves are equally good, this whole thing falls apart.) The "level 0" case here is discovering how good that move is and doing it all the time. Then, you will catch on and know that I'm likely to do that move a lot (yomi layer 1), so you'll need a counter move (we'll call it "c1"). You've stopped me from doing m. You've shut me down. I need a way to stop you from doing c1. I need a counter to your counter, or "c2".

Now you don't know what to expect from me anymore. I might do m or I might do c2. Interestingly, I probably want to do m, but I just do c2 to scare you into not doing c1 anymore. Then I can sneak in more m.

You don't have adequate choices yet. I can alternate between m and c2, but all you have is c1. You need a counter to c2, which we'll call c3. Now we each have two moves.

Player Options

Me You

Yomi Layer

Now I need a counter to c3. The tendency for game designers might be to create a c4 move, but it's not necessary. The move m can serve as my c4. Basically, if you expect me to do my counter to your counter (rather than my original good move m), then I don't need a counter for that; I can just do go ahead and do the original move-if the game is designed that way. Basically, supporting moves up to yomi layer 3 is the minimum set of counters needed to have a complete set of options, assuming yomi layer 4 wraps around back to layer 0.