In his article on Virtual Environments (a part of his VFSM specification method) Ferdinand Wagner describes some new ways of thinking about Boolean Algebra as a software design tool. On page 4 of this PDF article, when describing operators in his system he says this:

Control statements need Boolean values. Hence, the names must be used to produce Boolean results. To achieve this we want to combine them together using Boolean operators. There is nothing wrong with usage of AND and OR operators with their Boolean meaning. For instance, we may write:

DI_ON OR AI_LARGER_THAN_8.1 AND TIMER_OVER

to express the control situation: digital input is on or analog input is larger than 8.1 and timer is over.

We cannot use the NOT operator, because the result of the Boolean negation makes sense only for true Boolean values. The result of, for instance,

NOT (AI_LARGER_THAN_8.1)

would be ambiguous.

If "AI_LARGER_THAN_8.1" is acceptable, why would he consider "NOT (AI_LARGER_THAN_8.1)" to be ambiguous?