Part of these conditions concerns the form (syntax ) of a knowledge-representation formalism;
others concern its meaning (semantics). Unfortunately, it turns out that there is not a
single knowledge-representation formalism which meets all of the requirements mentioned.
In particular, the issues of expressive power of a formalism and its efficient interpretation
seem to be conflicting. However, as we shall see in the following chapters, by restricting
the expressive power of a formalism (in such a way that the domain knowledge can still be
represented adequately), we often arrive at a formalism that does indeed permit efficient
interpretation.