James Blish was one of the sf field's first important critics, as well as one of the more supremely intellectual practitioners of space opera. Perhaps his most important novel, A Case of Conscience deals with the ethical dilemmas of a Jesuit theologian confronted with a world of apparently unfallen aliens; it forms a loose thematic sequence After Such Knowledge with the historical novel Doctor Mirabilis and two novels of Satanism run apocalyptic Black Easter and The Day after Judgement. His space opera sequence, Cities in Flight, follows the adventures of New York as, powered with anti-gravity 'spindizzies', it tours a galaxy not entirely friendly to humans in pursuit of work and eventually is a key catalyst in humanity's, and the galaxy's, transcendence. The fact that most of the stories' science is entirely outmoded in no way damages its strong sense of the humane and the vast. Blish wrote some of the field's best early treatments of genetic manipulation in the short stories collected as The Seedling Stars. His sometimes cold and brightly clever fiction hides a strongly emotional heart; he produced some of 40s and 50s sf's most telling characters and set piece scenes.