Why the difference between androids and real human beings can be told?

Existentialists believe that human beings are born without an essence because the identity or value of human beings is determined by themselves. In the novel “Do Androids Dream of Electric Sheep?”, this concept of Existentialism is clearly presented. Human beings each develops their own identity which is distinct from those of others. For instance, in this novel, John Isidore is a representative character who processes obvious identity. He has his own belief in Mercerism and is able to determine which action to make based on different circumstances. When John Isidore is forced to make a phone call to Mrs. Pilsen, the owner of the dead cat, he could logically finish the conversation. When Isidore’s boss Sloat suggested to give a check as a compensation to Mrs. Pilsen, Isidore brought up the idea to compensate Mrs. Pilsen an electric duplicate of the cat, which turned out to be a successful proposal. Why human beings make decisions differently is exactly that they have different identity. If the thoughts of human beings were programmed, their responses to a certain circumstance would be similar. By contrast, androids show ambiguous identity due to that they are programmed. Androids are born with an essence which is to be the servants of human beings. Also, the Nexus-6 androids also have the same mission to eliminate human beings. They make decisions not because they personally want to do so, but because they are programmed to do the exact way of behaving. That’s the reason why the Voigt-Kampff test works out. When Rick was testing Racheal by saying that his briefcase was made of human babyhide, Racheal had a reaction but it was too late. Normally, human beings will response to this sentence instinctively, because they determine their own thoughts. However, Racheal, who was proved to be an android, was programmed to have the correct reaction in order to pretend that she was a human being. To conclude, the concept of essence indeed plays an important role in distinguishing human beings and androids.