various objects and parameters of the world (such as names of patients, quantities of medicament, etc.).
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> The multitude of models of $S$ reflects uncertainty regarding actual values of some of these parameters. For instance, as long as neither a final diagnosis nor a treatment of a patient $A$ is determined, there are, say, two possible diagnoses: $D_1$ (with possible treatments $T_1$ or $T_2$) or $D_2$ (with $T_3$ or $T_4$). So $MOD(S)$ may contain four different models, each including $D_1\; T_1$ or $D_1\; T_2$ or $D_2\; T_3$ or $D_2\; T_4$.
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> A set of values of all parameters of the world (including those not presented in $S$) determines its *state*. With regard to patient $A$ the hospital world has at least four possible states, each represented in $S$ by one of the four models mentioned above.
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> Since the available knowledge of the real world is incomplete in general, it may happen that a model $\mu$ of $S$ represents several states of the world that differ in the reality but are indistinguishable from the point of view of the information presented in $S$. For $S$ the states represented by $\mu$ constitute an equivalence class of possible states 