| Chapter 4: Kinds |

| 4.6. New either/or properties

Properties can only be given to something if its kind will permit that: we cannot make a chair "dark" or a room "portable", for instance. When creating kinds, then, we need a way to specify the properties which that kind of thing will allow, and this will need to include new properties of our own devising.

> A dead end is either secret or ordinary.

This creates just one new property, not two, as the names are taken as the two states of a single either/or property: secret means not ordinary, ordinary means not secret. Alternatively, we could just say:

> A dead end can be secret.

in which case the opposite of "secret" would be "not secret".

Note that we are free to add to the definitions of kinds which already exist, not just those we have created ourselves: for instance,

> A room is either indoors or outdoors.

creates a new property which all rooms either possess or don't. And so on. If we make the above definitions then by default, any dead ends created will be "ordinary" and any rooms created will be "outdoors": that is, in the absence of other information it is assumed that a new either/or property will not be held. We could reverse this behaviour by adding:

> A dead end is usually secret. A room is usually indoors.

(It is not quite true that a property can only be held be something if the kind permits it, because in exceptional cases we can give special permission. For instance, the "openable" property is normally only applicable to doors and containers. Here we give special leave for it to apply to a specific thing:

> The umbrella is carried by the player. The umbrella can be openable. The umbrella is openable.

In practice, this possibility is only occasionally needed.)

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| Example Change of Basis Implementing sleeping and wakeful states.

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