Abstract
It is argued that contraposition is valid for a class of natural language conditionals, if some
modifications are allowed to preserve the meaning of the original conditional. In many
cases, implicit temporal indices must be considered, making a change in verb tense
necessary. A suitable contrapositive for implicative counterfactual conditionals can also
usually be found. In some cases, the addition of certain words is necessary to preserve
meaning that is present in the original sentence and would be lost or changed in the
contrapositive without them. A distinction is made between adding new meaning and
adding new words to preserve existing meaning. For concessive conditionals and relevance
conditionals, however, no valid contrapositive can be found. They do not belong to the
class of contraposable conditionals, which can be independently defined. Difficult cases are
also discussed in which the contradictory of the consequent semantically entails the truth
of the antecedent. In such cases the content of the antecedent is implicit in the meaning of
the consequent. Contraposition becomes possible if what is implicit in the original
consequent is made explicit in the contrapositive antecedent.