The central theme of this excellent work, which was, of course, made into the film Bladerunner starring Harrison Ford, is the nature of humanity. What makes us human? And in particular, when we are in a world populated by humans and androids who resemble humans so perfectly and even have emotions and are given their own past, like Rachel, what separates us from androids? We, as humans, are able to dream of one day having possessions or reaching positions of happiness and contentment, but are androids able to have similar dreams, just as Deckard dreams of having his own electric sheep? And if they are, what really does make us different from artificially formed life?
Thus the question of the title goes to the heart of the theme of humanity in the play and how we define it. Throughout it all, the worrying notion of Deckard himself being a replicant questions our own basis of how we define ourselves and severely limits the sense in which we can consider ourselves to be unique.