How does Terry Pratchett use imagery in The Last Continent: A Discworld Novel?

In the section, Time Travel, Stibbons struggles to convince his colleagues that they must exercise extreme care not to change the course of history. He uses the image of how someone stepping on an ant could snowball into catastrophic events. The older wizards wonder if the ant had not been meant to be stepped on to bring about current events. When Stibbons uses the classical argument that if one accidentally killed or prevented the birth of one's own grandfather, one would not exist. The wizards take the image literally and get bogged down over why anyone would want to kill his kindly grandfather.