In mathematical logic a sentence is called independent from a theory when the theory alone does not settle the sentence's truth: neither the sentence nor its negation can be derived from the theory's axioms. This notion is usually stated for formal systems such as a first-order theory. Independence is a statement about provability relative to a given set of axioms, not about whether the statement is decidable by an algorithm.
Formal perspectives
There are two commonly used but equivalent viewpoints. Syntactically, a sentence φ is independent of a theory T when T ⊬ φ and T ⊬ ¬φ: neither φ nor its negation is provable from T. Semantically, φ is independent of T when there exist two models, one satisfying T∪{φ} and another satisfying T∪{¬φ}; thus T does not imply φ in all models. Completeness theorems link these views for many formal systems.
How independence is established
- Relative consistency/model construction: exhibit a model of T+φ and a model of T+¬φ, often by building models with different combinatorial or set-theoretic properties.
- Forcing: a method introduced to construct models of set theory in which particular sentences hold or fail; famously used to settle independence results about the continuum.
- Conservativity and extensions: show that adding an axiom does not introduce contradictions or that it is not derivable from existing axioms.
Examples and distinctions
Classical highlights include Gödel's incompleteness results, which show that sufficiently strong theories (like Peano arithmetic) contain true sentences that are unprovable within the theory, yielding independence between the theory and particular Gödel sentences. In set theory, the Continuum Hypothesis and the Axiom of Choice are famous examples: Gödel and Cohen demonstrated that these statements are independent of the standard Zermelo–Fraenkel axioms (ZFC) in the sense of provability and model existence. Independence is not the same as algorithmic decidability; a sentence can be provably independent from a theory while being algorithmically decidable in some other sense.
Significance and consequences
Independence results illuminate the limits of axiomatic systems and guide choices of additional axioms or principles in mathematics. They show where mathematical practice may legitimately add hypotheses to resolve questions, and they motivate study of relative consistency and alternative foundations. In logic and foundations, independence is a key tool for understanding the expressive and deductive power of theories.