Rule of Implication/Proof Rule
Proof Rule
The Rule of Implication is a valid deduction sequent in propositional logic.
As a proof rule it is expressed in the form:
- If, by making an assumption $\phi$, we can conclude $\psi$ as a consequence, we may infer $\phi \implies \psi$.
It can be written:
- $\displaystyle {\begin{array}{|c|} \hline \phi \\ \vdots \\ \psi \\ \hline \end{array} \over \phi \implies \psi} \to_i$
Tableau Form
Let $\phi$ and $\psi$ be two propositional formulas in a tableau proof.
The Rule of Implication is invoked for $\phi$ and $\psi$ in the following manner:
Pool: | The pooled assumptions of $\psi$ | |||||||
Formula: | $\phi \implies \psi$ | |||||||
Description: | Rule of Implication | |||||||
Depends on: | The series of lines from where the assumption $\phi$ was made to where $\psi$ was deduced | |||||||
Discharged Assumptions: | The assumption $\phi$ is discharged | |||||||
Abbreviation: | $\text{CP}$ or $\implies \mathcal I$ |
Explanation
The Rule of Implication can be expressed in natural language as:
- If by making an assumption $\phi$ we can deduce $\psi$, then we can encapsulate this deduction into the compound statement $\phi \implies \psi$.
Also known as
The Rule of Implication is sometimes known as:
- The rule of implies-introduction
- The rule of conditional proof (abbreviated $\text{CP}$).
Technical Note
When invoking the Rule of Implication in a tableau proof, use the {{Implication}}
template:
{{Implication|line|pool|statement|start|end}}
where:
line
is the number of the line on the tableau proof where the Rule of Implication is to be invokedpool
is the pool of assumptions (comma-separated list)statement
is the statement of logic that is to be displayed in the Formula column, without the$ ... $
delimitersstart
is the line of the tableau proof where the antecedent can be foundend
is the line of the tableau proof where the consequent can be found
Sources
- 1964: Donald Kalish and Richard Montague: Logic: Techniques of Formal Reasoning ... (previous) ... (next): $\text{I}$: 'NOT' and 'IF': $\S 3$
- 1965: E.J. Lemmon: Beginning Logic ... (previous) ... (next): $\S 1.2$: Conditionals and Negation
- 1973: Irving M. Copi: Symbolic Logic (4th ed.) ... (previous) ... (next): $3.5$: The Rule of Conditional Proof
- 1980: D.J. O'Connor and Betty Powell: Elementary Logic ... (previous) ... (next): $\S \text{II}$: The Logic of Statements $(2): \ 7$: Conditional proof
- 2000: Michael R.A. Huth and Mark D. Ryan: Logic in Computer Science: Modelling and reasoning about systems ... (previous) ... (next): $\S 1.2.1$: Rules for natural deduction