An epistemic state is the state of knowledge about an assertion in a system that receives information from multiple sources (Belnap 1977).

There are four states:

• $\bot$ (unattested): no source has said anything about this assertion.
• $t$ (affirmed): at least one source says true; no source says false.
• $f$ (denied): at least one source says false; no source says true.
• $\top$ (contested): at least one source says true AND at least one says false.

These four states form the four-valued bilattice $\{\bot, t, f, \top\}$, which is the twist product $\mathbf{2} \otimes \mathbf{2}$ (Fitting 1991). The first component tracks positive evidence; the second tracks negative evidence.

The states carry two orderings. The truth ordering ranks by assertion strength: $f \leq \bot \leq t$ and $f \leq \top \leq t$. The knowledge ordering ranks by information content: $\bot \leq t \leq \top$ and $\bot \leq f \leq \top$.

Epistemic states are not truth values in the classical sense. A classical system has two states (true, false). A three-valued system adds “unknown.” Belnap’s system adds “contested” — the state where contradictory information is present, which is not the same as unknown. A system can have both much information ($\top$) and no resolution.