Material implication (rule of inference)

The material implication rule may be written in sequent notation:

${\displaystyle (P\to Q)\vdash (\neg P\lor Q)}$

where ${\displaystyle \vdash }$ is a metalogical symbol meaning that ${\displaystyle (\neg P\lor Q)}$ is a syntactic consequence of ${\displaystyle (P\to Q)}$ in some logical system;

or in rule form:

${\displaystyle {\frac {P\to Q}{\neg P\lor Q}}}$

where the rule is that wherever an instance of "${\displaystyle P\to Q}$" appears on a line of a proof, it can be replaced with "${\displaystyle \neg P\lor Q}$";

or as the statement of a truth-functional tautology or theorem of propositional logic:

${\displaystyle (P\to Q)\to (\neg P\lor Q)}$

where ${\displaystyle P}$ and ${\displaystyle Q}$ are propositions expressed in some formal system.

Suppose we are given that ${\displaystyle P\to Q}$. Then, since we have ${\displaystyle \neg P\lor P}$ by the law of excluded middle, it follows (arguing by cases) that ${\displaystyle \neg P\lor Q}$.

Suppose, conversely, we are given ${\displaystyle \neg P\lor Q}$. Then if ${\displaystyle P}$ is true that rules out the first disjunct, so we have ${\displaystyle Q}$. In short, ${\displaystyle P\to Q}$.[3] However if ${\displaystyle P}$ is false, then this entailment fails, because the first disjunct ${\displaystyle \neg P}$ is true which puts no constraint on the second disjunct ${\displaystyle Q}$. Hence, nothing can be said about ${\displaystyle P\to Q}$. In sum, the equivalence in the case of false ${\displaystyle P}$ is only conventional, and hence the formal proof of equivalence is only partial.

This can also be expressed with a truth table:

An example is:

We are given the conditional fact that if it is a bear, then it can swim. Then all 4 possibilities in the truth table are compared to that fact.

1st: If it is a bear, then it can swim — T

2nd: If it is a bear, then it can not swim — F

3rd: If it is not a bear, then it can swim — T because it doesn’t contradict our initial fact.

4th: If it is not a bear, then it can not swim — T (as above)

Thus, the conditional fact can be converted to ${\displaystyle \neg P\vee Q}$, which is "it is not a bear" or "it can swim", where ${\displaystyle P}$ is the statement "it is a bear" and ${\displaystyle Q}$ is the statement "it can swim".