The series is noted for its diverse and believable non-humanoid alien life forms, and for its pacifist philosophy. White chose the hospital setting as a way to generate dramatic tension without violence, and because in his youth he wanted to be a doctor but had to go to work. Some commentators have praised the whole series, while others have thought there was a decline after the sixth book. One reviewer described the last book as "in a very positive way, a throwback to an earlier era in science fiction".