|If P, then Q. Not Q, therefore not P.|
Denying the consequent.
A valid form of argument (unlike affirming the consequent and denying the antecedent).
1. If A, then B.
2. Not B.
3. Therefore, Not A.
Modus tollens is related to modus ponens. They both have a premise that is a conditional statement. The most important difference lies in the negation of the last two lines.
See also Modus Ponens