The discussion of rule-based systems in Chapter 2 assumed that we live in a clear-cut world where every hypothesis is either true, false, or unknown. Furthermore, it was pointed out that many systems make use of the closed-world assumption, whereby any hypothesis that is unknown is assumed to be false. We were then left with a binary system where everything is either true or false. While this model of reality is useful in many applications, real reasoning processes are rarely so clear-cut. Referring to the example of the control of a power station boiler, we made use of the following rule:
if transducer_output is low
then water_level becomes low. ...