Natural Deduction System in Propositional Logic

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5 Questions

What does the Implication Introduction rule allow us to introduce?

A and B simultaneously

What is an assumption in natural deduction?

A premise

How is Conditional Proof used in proving a statement A → B?

Assuming A and proving B

In the provided example of natural deduction, what is achieved in step 8?

(A ∧ B) → (C ∧ D)

What is the purpose of using natural deduction systems?

To develop logical proofs intuitively

Study Notes

Natural Deduction System: A Methodical Approach to Reasoning

In the realm of mathematical logic, a natural deduction system is a rigorous approach to constructing logical proofs. This method, which originated in the mid-20th century, offers a compelling alternative to traditional formal systems like Hilbert-style proofs, providing a more intuitive and direct means of demonstrating the validity of logical statements.

Propositional Logic

Natural deduction systems primarily deal with propositional logic, which is the study of the relationships between propositions (statements that are either true or false). Propositional logic is the foundation upon which natural deduction rests, providing the building blocks for constructing proofs.

Proof Strategies

The most critical component of a natural deduction system is the proof strategy. This strategy guides the construction of a valid proof by following a specific set of rules that dictate the order and structure of the steps. These rules are known as inference rules.

Inference Rules

Inference rules define the legal moves that can be made when constructing a proof. These rules are derived from the connectives and logical operators found in propositional logic. A few essential examples include:

  • Modus Ponens: This rule states that if we have premises A and A → B, then we can infer B.
  • Disjunction Elimination: This rule allows us to prove one of the two disjuncts (either A or B) given A ∨ B.
  • Implication Introduction: This rule allows us to introduce an implication (A → B) if we can prove A and B simultaneously.


An assumption (also known as a hypothesis or premise) is a proposition that is temporarily accepted as true for the purpose of constructing a proof. Assumptions are denoted with a horizontal line, often referred to as the turnstile (⊸), and can be cancelled by proving the negation of the assumption.

Conditional Proof

Conditional proof is a method used to prove a statement of the form A → B. It involves assuming A is true and constructing a proof of B, after which the proof of A → B is completed by discharging the assumption. Conditional proof allows us to construct proofs that resemble the natural flow of our reasoning, making the proof more intuitive and easier to follow.

Example of Natural Deduction

Let's consider a simple example to illustrate the natural deduction system in action:

  1. A ∧ B (Assume A and B are true)
  2. A → C (A premise)
  3. A (From assumption 1, using conjunction elimination)
  4. C (From 2 and 3, using modus ponens)
  5. B → D (A premise)
  6. B (From assumption 1, using conjunction elimination)
  7. D (From 5 and 6, using modus ponens)
  8. A ∧ B → (C ∧ D) (From 4 and 7, using conjunction introduction)
  9. □ (End of proof)

In this example, we first assume A and B are true (step 1) and then use modus ponens to prove C (step 4) and D (step 7) from the given premises. Finally, we use conjunction introduction to show that A and B imply the statement C and D (step 8).

Natural deduction systems offer a powerful and versatile tool for developing logical proofs and have become a central part of mathematical reasoning and computer science. As we continue to explore the boundaries of logic and computation, the natural deduction system will undoubtedly remain an essential tool in our quest to understand and manipulate the fundamental principles of reasoning.

Explore the methodology of natural deduction in propositional logic, a foundational concept in mathematical reasoning. Learn about proof strategies, inference rules, assumptions, and conditional proof, all crucial components of constructing logical proofs within a natural deduction system.

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