A Case of Conscience is a science fiction novel by American writer James Blish, first published in 1958. It is the story of a Jesuit who investigates an alien race that has no religion yet has a perfect, innate sense of morality, a situation which conflicts with Catholic teaching. The story was originally published as a novella in 1953, and later extended to novel-length, of which the first part is the original novella. The novel is the first part of Blish's thematic "After Such Knowledge" trilogy and was followed by "Doctor Mirabilis" and both "Black Easter" and "The Day After Judgment" (two novellas that Blish viewed as together forming the third volume of the trilogy).