Kennedy: The Implicatures

the statement "The president was assassinated" not only suggests that "The president is dead" is true, but requires that it be true. The first sentence could not be true if the second were not true; if the president were not dead, then whatever it is that happened to him would not have counted as a (successful) assassination. Similarly, unlike implicatures, entailments cannot be cancelled; there is no qualification that one could add to "The president was assassinated" which would cause it to cease entailing "The president is dead" while also preserving the meaning of the first sentence.