3.4 Indirect Proof Methods: Conditional and Reductio ad Absurdum
3 min read•july 22, 2024
Indirect proof methods are powerful tools in logic, allowing us to prove statements that might be challenging to demonstrate directly. These methods, including and , work by assuming the opposite or a related condition.
By exploring these techniques, we gain a deeper understanding of logical reasoning and expand our problem-solving toolkit. Indirect proofs complement direct methods, offering alternative approaches to tackle complex logical statements and arguments.
Indirect Proof Methods
Concepts of conditional proof and reductio
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Indirect proof methods prove a statement by considering its or a related conditional statement when a direct proof is difficult or not possible
Conditional proof (CP) method proves a statement of the form P→Q by assuming P is true and deriving Q from this , establishing the truth of P→Q if successful
Reductio ad absurdum (RAA) method, also known as proof by , assumes the negation of the statement to be proved and derives a contradiction from this assumption, establishing the truth of the original statement if successful
Application of conditional proof method
To prove a statement of the form P→Q using CP:
Assume P is true
Derive Q using logical inferences and the assumption P
Conclude that P→Q is true if successful
Example: Prove (A∧B)→(A∨C)
Assume A∧B is true
Infer A from A∧B (simplification)
Infer A∨C from A (addition)
Conclude (A∧B)→(A∨C) is true
Reductio ad absurdum for contradiction proofs
To prove a statement P using RAA:
Assume ¬P (the negation of P)
Derive a contradiction (e.g., Q∧¬Q) using logical inferences and the assumption ¬P
Conclude that P must be true if successful
Example: Prove A→(B→A)
Assume ¬(A→(B→A))
Infer A∧¬(B→A) (negation of implication)
Infer A (simplification)
Infer B∧¬A from ¬(B→A) (negation of implication)
Infer ¬A (simplification)
Contradiction: both A and ¬A are true
Conclude A→(B→A) is true
Direct vs indirect proof methods
Direct proof method proves a statement by deriving it directly from known premises and inference rules, starting with the premises and reaching the conclusion through a sequence of logical steps
Indirect proof methods (CP and RAA) prove a statement by considering its negation or a related conditional statement
CP assumes the antecedent of a conditional statement and derives the consequent
RAA assumes the negation of the statement to be proved and derives a contradiction
Indirect proofs are useful when a direct proof is difficult or not possible
Both direct and indirect proofs rely on logical inferences and the rules of propositional logic
Selection of appropriate proof methods
Direct proof is suitable when the statement can be easily derived from known premises and inference rules and is not a conditional or a negation
Conditional proof is suitable when the statement to be proved is a conditional (P→Q) and it is easier to assume the antecedent (P) and derive the consequent (Q)
Reductio ad absurdum is suitable when the statement to be proved is a negation or a statement that leads to a contradiction and it is easier to assume the negation of the statement and derive a contradiction
Multiple proof methods may be applicable in some cases, and the choice depends on the structure of the statement and the ease of deriving the conclusion (e.g., proving A∨¬A using direct proof or RAA)