of the world called a *possible world* (denoted $w$ in the sequel).
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> $S$ describes the world *faithfully* in the sense that every possible state of the world belongs to a possible world represented by a model of $S$, and every model of $S$ represents a non-empty set of possible states of the world. From the point of view of $S$ the real world appears as a set $W$ of possible worlds such that there is a bijection between $MOD(S)$ and $W$.

A user of $S$ having a query whether a formula $F$ is true in the present state of the world applies to $S$ and expects to get an answer based on the information stored in $S$. If $F$ (or $\neg F$) is a logical consequence of $S$ then $S$ contains *complete* information about $F$. In this case $F$ is true (false) in all models of $S$ and all possible worlds. Hence $F$ is certainly true (false) in the present state. However, if neither $F$ nor $\neg F$ follows from $S$ then the information in $S$ regarding $F$ is *incomplete* and does not facilitate derivation of a definite answer to the query. But this does not diminish the 