# Rule of Assumption

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## Contents

## Sequent

The **rule of assumption** is a valid deduction sequent in propositional logic.

### Proof Rule

- An assumption may be introduced at any stage of an argument.

### Sequent Form

For structure-technical reasons, the **rule of assumption** is symbolised by the sequent:

- $p \vdash p$

In this form it is usually referred to as the Law of Identity.

### Boolean Interpretation

The truth value of a propositional formula $\mathbf A$ under a boolean interpretation $v$ is given by:

- $v \left({\mathbf A}\right) = \begin{cases} T & : v \left({\mathbf A}\right) = T \\ F & : v \left({\mathbf A}\right) = F \end{cases}$

## Explanation

It does not matter whether the assumption is true -- all we are concerned about is making sure that any conclusion based on the assumptions made is as the result of a valid argument.

The introduction of an assumption $\phi$ into an argument by means of the **Rule of Assumption** can be interpreted in natural language as:

- "Suppose it were true that $\phi$"

or:

- "What if $\phi$ were true?"

## Also known as

Some sources refer to the Rule of Assumption as the **rule of assertion**.