These difficulties raise concerns regarding genre boundaries: while Christian science fiction has been identified as a specific market into which stories can be sold, Doyle has questioned whether or not books that are, at times, classified in this subgenre truly fit. In examining Christian apocalyptic fiction, Doyle notes that it is often classified as Christian science fiction, but argues that this classification is inappropriate. While both may employ scientific themes, Christian apocalyptic fiction is not, as he describes it, "scientifically minded", arguing that the authors tend to respond to scientific problems "with biblical authority, prophetic interpretation, and fundamentalist ideas of human identity instead of rational argument, scientific method, and humanistic thought". Doyle sees Brian Caldwell's "We All Fall Down" as an exception to his argument, suggesting that (despite being a work of Christian apocalyptic fiction) it is the sort of work that he would like to see classified as science fiction.