inconsistency

Inconsistency refers to a lack of coherence between elements that should be compatible. In everyday language it can describe contradictory beliefs, statements, or actions. In logic and formal theories, an inconsistency occurs when a set of propositions entails both a statement and its negation, making it possible to derive any statement from them. The term comes from Latin inconsistentia, meaning not being in agreement.

In formal logic, the presence of a contradiction means the theory is trivial under the principle of explosion: from falsehood, anything follows. Most classical logics treat inconsistency as a sign of error, whereas paraconsistent logics are designed to tolerate some contradictions without explosive consequences. Detecting inconsistencies involves proof methods such as truth tables, natural deduction, resolution, or semantic tableaux; counterexamples and model checking are common tools.

Outside pure logic, inconsistencies appear in science, law, policy, and data analysis when observed results conflict with theories, measurements, or rules. Conflicting data entries, incompatible standards, or changing requirements can create practical inconsistencies. Resolving them often requires clarifying context, reevaluating assumptions, or reconciling competing models; in some cases inconsistencies are temporary and were due to incomplete information.

Managing inconsistency also involves processes such as version control, documentation, and transparency about uncertainties. In philosophy and epistemology, inconsistency raises questions about justification, belief revision, and the limits of knowledge. In short, inconsistency is a multi-faceted concept addressing coherence among statements, data, and theories across disciplines.