In fact, you might have heard recently about AlphaZero which can beat humans. Well, it's essentially superhuman in chess, Go, and shogi, all using the same algorithm. So, what is it about imperfect information games that are so difficult? One of the major challenges, not the only one, but one of the major ones, is that in an imperfect-information game, the optimal strategy for a subgame, for part of the game, cannot be determined in isolation. It cannot be determined using information in just that subgame alone.