In Do Androids Dream of Electric Sheep by Philip K. Dick, humans are distinguished from androids by one single criterion: empathy. In a post-apocalyptic world where replicants of humans are stronger, physically and intellectually, humans begin to cling the idea of compassion as the one distinguishing factor between humans and androids. .
The test performed to determine humanity measures empathic response to uncomfortable social situations and cruelty towards animals. Yet even in that test, the difference between the new NEXUS-6 type androids and normal humans is manifested solely in the delayed nerve responses in the eyes, which varies between the two "species" by a fraction of a second. In that split second, the pre-programmed (or possibly developed) compassion and the "real" compassion are distinguished, and, in the case of all the androids presented in the book, can mean life or death. However there is one setback: the androids have not been programmed to feel empathy, yet they have developed it. In other words, the replicants actually can feel sympathy and compassion, but their response time (measured in milliseconds) is minutely slower. .
Ironically, the humans themselves simulate feelings with manufactured "mood organs," which provide emotion with a click of a button. The one distinguishing quality of humans has been taken over by a machine. Even the protagonist of the story, Rick Deckard, uses a mood organ to change his mood to a more positive one. .
The other manufactured machine to simulate real feelings is an "empathy box." The empathy boxes allow a large portion of the population to feel a sort of "oceanic" consciousness, a way of exploiting the empathy that has such significance to humans. This fusing is the religion in the book, called Mercerism. It is the true testimate to how much the difference between the fabricated replicants and humans matters to humanity. Mercer, the character that the populace becomes, climbs up a hill as rocks are thrown down at him.