Theories

Definition

In mathematical logic, theories are sets of sentences in a formal language that are closed under logical consequence. They provide a structured framework to explore mathematical truths and relationships, allowing for the study of consistency, completeness, and decidability within a logical system. Theories play a crucial role in understanding the Completeness Theorem, which asserts that if a statement is true in all models of a theory, then it can be proven within that theory.

5 Must Know Facts For Your Next Test

1. Theories consist of axioms and theorems that can be derived from those axioms using logical reasoning.
2. The Completeness Theorem shows that if a statement is true in all models of a theory, there exists a proof for it using the axioms of that theory.
3. Not every collection of sentences forms a theory; for it to qualify as a theory, it must be closed under logical consequence.
4. A theory can be consistent but not complete, meaning there could be true statements that cannot be proven within the theory.
5. Different theories can have different models; two theories might be consistent and complete yet have no models in common.

Review Questions

• How do theories relate to models in mathematical logic?
  • Theories and models are intimately connected in mathematical logic. A theory consists of sentences that articulate certain truths about mathematical structures, while models are specific interpretations where those sentences hold true. Each model demonstrates how the axioms and derived statements of a theory can manifest in particular cases. Thus, models serve as evidence for the validity of a theory by illustrating how its statements apply in real-world scenarios or abstract constructions.
• Discuss the implications of the Completeness Theorem on the understanding of theories.
  • The Completeness Theorem has profound implications for our understanding of theories in mathematical logic. It states that if a statement is valid in every model of a theory, then there exists a formal proof for that statement within the theory itself. This highlights the powerful relationship between syntactic proofs and semantic truth, showing that if something is logically true, it can be proven through formal methods. Consequently, this establishes an essential link between what we can know within a theoretical framework and the reality described by its models.
• Evaluate how consistency and completeness interact within theories and their significance in mathematical logic.
  • The interaction between consistency and completeness is crucial for the integrity and utility of theories in mathematical logic. A consistent theory ensures that no contradictions arise from its axioms, allowing for reliable reasoning. Completeness, on the other hand, indicates that all truths expressible within the language of the theory can be proven using its axioms. The significance lies in Gödel's Incompleteness Theorems, which show that in any sufficiently powerful and consistent theory, there will always be true statements that cannot be proven within that framework. This realization impacts how mathematicians approach foundational questions about truth and proof in logic.

Related terms

Models: Models are interpretations of a theory where the sentences of the theory hold true. They serve as concrete instances that demonstrate the validity of a theory's axioms and theorems.

Axioms: Axioms are foundational statements or propositions within a theory that are accepted without proof and used to derive other statements.

Consistency: Consistency refers to the property of a theory where no contradictions can be derived from its axioms. A consistent theory ensures that there is at least one model where its axioms hold true.