Modus tollens is a logical reasoning rule used in deductive arguments. It states that if a conditional statement is true, and the consequent (the "then" part) is false, then the antecedent (the "if" part) must also be false. This can be expressed as: if P implies Q, and Q is false, then P is also false. For example, consider the statement: "If it is raining, then the ground is wet." If we observe that the ground is not wet, we can conclude that it is not raining. This reasoning helps in drawing valid conclusions based on the relationships between conditions.