Where was the grandfather paradox first mentioned?

The grandfather paradox is a paradox of time travel in which inconsistencies emerge through changing the past. The name comes from the paradox's common description: a person travels to the past and kills their own grandfather before the conception of their father or mother, which prevents the time traveller's existence. Despite its title, the grandfather paradox does not exclusively regard the contradiction of killing one's own grandfather to prevent one's birth. Rather, the paradox regards any action that alters the past, since there is a contradiction whenever the past becomes different from the way it was.

The grandfather paradox was alluded to in written stories as early as 1929. In 1931 it was described as "the age-old argument of preventing your birth by killing your grandparents" in a letter to American science fiction magazine Amazing Stories. Early science fiction stories dealing with the paradox are the short story Ancestral Voices by Nathaniel Schachner, published in 1933, and the 1944 book Future Times Three by René Barjavel, although a number of other works from the 1930s and 1940s touched upon the topic in various degrees of detail.