Databases Reference
In-Depth Information
Turning the ignition switch on is one action in a complex of conditions required
to start the engine. One of the events might be used to represent the collection
of equal grain sized events; or, a higher-level granule might be specified with
the understanding that it will invoke a set of finer-grained events.
In terms of nested granules, the largest-grained event is the sole causal
element:
Turn on the ignition switch.
The complex of other elements repre-
sents the finer-grains. These elements in turn could be broken down into still
finer-grains; for example, “available fuel” can be broken down into:
fuel in tank, working fuel pump, intact fuel lines, . . .
Sometimes, it is enough to know what happens at a large-grained level; at
other times it is necessary to know the fined grained result. For example, if
Bill believes that turning the ignition key of his automobile
causes
the
automobile to start.
It is enough if
Bill engages an automobile mechanic when his automobile does not
start when he turns the key on.
However,
The automobile mechanic needs to know a finer-grained view of an
automobile's causal complex than does Robin.
Instead of being concerned with all of the fined grained detail, a better ap-
proach may be to incorporate granulation using rough sets and/or fuzzy sets
to soften the need for preciseness. If needed, larger complexes can be decom-
posed into smaller complexes.
Hobbs [8] uses first order logic to describe his causal complexes. Pearl
[20] develops probabilistic causal networks of directed graphs (DAGs). The
causal complexes explicitly considered separately by Hobbs and Pearl have
a required structure that may be overly restrictive for commonsense causal
understanding, namely:
•
If
all
of the events in the causal complex appropriately happen, then the
effect will occur
•
There is nothing in the causal complex that is irrelevant to the effect
These requirements are probably too precise and extensive to be realized
in a commonsense world. Sometimes, only some of the events need to happen.
For example,
Someone may be able to save more money:
•
If their taxes are lowered or
•
If they earn more money.
