 He has included limited psi gifts (mind over matter) in some characters in his stories; like Gil Hamilton's psychic arm which can only reach as far as a corporeal arm could, though it can, for example, reach through solid materials and manipulate objects on the other side, and through videophone screens, or Matt Keller's ability to make people not notice him.Several of his stories predicted the black market in transplant organs ("organlegging").Many of Niven's stories—sometimes called the Tales of Known Space—take place in his Known Space universe, in which humanity shares the several habitable star systems nearest to the Sun with over a dozen alien species, including the aggressive feline Kzinti and the very intelligent but cowardly Pierson's Puppeteers, which are frequently central characters.