Logic question about implication Given the logic rule: When the weather is stormy - The fish aren't sleep. Why **can't** we deduce: When the fish sleep - The weather isn't stormy But, we **can** deduce: When the fish aren't sleep - the Weather is stormy.

The "logical rules" are :

> from "if $P$, then $Q$" deduce : "if not $Q$, then not $P$

and :

> from "if $P$, then not $Q$" deduce : "if $Q$, then not $P$.

Thus, if we assume that :

> When the weather is stormy - The fish aren't sleep

has the same meaning of :

> If the weather is stormy, then the fish aren't sleep,

it has the "logical form" :

> "if $P$, then not $Q$.

Thus, from it we **can** deduce :

> If the fish sleep, then the weather is not stormy.

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Instead, from "if $P$, then $Q$, we **cannot** deduce : "if $Q$, then $P$.