or short: modus tollens

the two modi (ponens and tollens) are two sides of the same medal:

We have a proposition such as: If A then B.

We could even say: If and only if A then follows necessarily B.

This proposition should not be limited by any other condition! [this is rarely the case in real life :-) ]

And there is no statement when we have if A then might B
since we can't infer anything about A from the existence of B and vice versa.

modus ponens infers from A that B,

modus tollens infers from not B that not A.

In real life we have examples which do not work:

If it is a bird[A] then it flies.

Sounds logical: but imagine an oistrich...this is real life in most cases we have few cases where we can formulate strict proposition. That is one reason because of which logic is mostly used and thaught in mathematics.