“机器人与外星人系列”序言相关部分:“The ending of my novel Foundation and Earth makes it conceivable that in the sequel I may introduce aliens and that R. Daneel will have to deal with them. That’s not a promise because actually I haven’t the faintest idea of what’s going to happen in the sequel, but it is at least conceivable that aliens may intrude on my close-knit human societies. ”