Theory

Definition

In mathematical logic, a theory is a set of sentences in a formal language that is closed under logical consequence. This means that if a sentence can be derived from the sentences in the theory using valid rules of inference, then that sentence is also included in the theory. Theories provide a framework for reasoning about mathematical structures and can be used to explore relationships between different systems, particularly in the context of completeness and equivalence.

5 Must Know Facts For Your Next Test

1. A theory can be seen as a collection of propositions that are believed to be true and can be used to derive further truths.
2. Henkin's proof of the completeness theorem shows that if a theory is consistent, then it has a model, establishing a strong connection between theories and models.
3. Two theories are said to be elementarily equivalent if they satisfy the same first-order sentences, regardless of their specific axioms or structures.
4. The process of proving the completeness theorem involves constructing models for theories and demonstrating that all syntactically valid sentences are semantically valid.
5. The concept of elementary equivalence highlights how different theories can express similar mathematical structures while differing in their axioms.

Review Questions

• How does the concept of a theory relate to Henkin's proof of the completeness theorem?
  • Henkin's proof establishes that any consistent theory has an associated model. This relationship illustrates that theories not only contain axioms but also lead to meaningful interpretations within models. In essence, if you have a theory that doesn't produce contradictions, Henkin showed you can always find a structure where all those axioms hold true, bridging the gap between syntax and semantics.
• What role do axioms play in defining a theory, and how does this relate to elementary equivalence?
  • Axioms serve as the foundational elements from which all other propositions in a theory are derived. When considering elementary equivalence, two theories can differ in their axioms yet still satisfy the same first-order sentences. This means they can model similar structures or properties while originating from different starting points, showing how flexible mathematical logic can be with its frameworks.
• Evaluate how theories contribute to understanding mathematical structures and their relationships within logical systems.
  • Theories act as powerful tools for encapsulating knowledge about mathematical structures and their interrelations. By examining how different theories can express similar properties through their axioms, one gains insights into the underlying principles governing those structures. This evaluation not only enhances comprehension of specific systems but also deepens understanding of the broader landscape of mathematical logic and its applications across various domains.

Related terms

Model: A model is a mathematical structure that satisfies the sentences of a theory, demonstrating the truth of those sentences within that structure.

Axioms: Axioms are the foundational statements or assumptions within a theory from which other sentences can be derived through logical reasoning.

Consistency: A theory is consistent if it does not contain any contradictions; that is, there are no sentences such that both the sentence and its negation can be derived from the theory.