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Our minds work in mysterious ways.

They don't accept this world and move on. They catalogue it, characterize it, identify it, and constantly re-affirm it as 'a world'. When a player is faced with another world, his subconscious mind springs into action and either identifies it as a believable world, or identifies it as false, and then feels it's 'wrong'.

As designers we need to find the characteristics under which our minds catalogue 'a world'. We need to make sure that the world we build fits those characteristics. If it fits – we've built a believable world. If it doesn't – we've built a world that will not be accepted by most players.

In the next few weeks, this Storytelling Tips column is going to cover the various characteristics that the players' minds recognize as real. There are actually surprisingly few.

We'll begin with the simplest of them all: Worlds need doors that never open.

'Doors' in this case are metaphoric. Every world you create needs something that can't be solved, can't be seen, or can't be understood. It can be a story that isn't told, a door that never opens, a window you can't see through, a window you can see through but shows you a place you can never be in, an alley that wasn't taken, a person that remains a mystery, a mystery that remains unsolved, or anything else that is unexplored, unopened, or unknown even if you explore the entire game through and through.

Our subconscious minds recognize this world as always having 'more': more to see, more to discover, more to know, more places to go to, etc. Our subconscious minds see all around paths that will never be taken, horizons that will never be traveled to, people we will never talk to, doors that will never open, and avenues that won't be taken. All these fall into the metaphoric definition of 'closed doors'. Our minds subconsciously catalogue 'a world' as something that has many closed doors.

The second our minds, after having played a beautiful game with a beautiful world, resolve everything and opens the last door to be opened - at that second the game's world will lose its beauty and believability.

(Note that the rules of storytelling are different from the rules of world-building. When you solve all the avenues you've opened in storytelling, the player gets a feeling of perfection. However, the world the story takes place in must have some avenues, even avenues hinted at, that can never be closed – that lends that world a feeling of perfection.)

When creating a world in a game, it's tempting to reward the players who pass everything and explore every corner with little nuggets that show what's behind every unexplored door. Yes, it's very rewarding to players to know they've achieved what few will and to learn a secret. But even these players will feel cheated and not know why if there are no doors whatsoever (even ones only hinted at) that remain closed.

Even for expert players you must leave at least one thing that they couldn't do or couldn't touch or couldn't solve – shuttered windows, a criminal that was never caught but only hinted at, etc. etc. etc.

Do that, and your world will be believable.

Next week we'll tackle more criteria our subconscious minds require for believable worlds.