The central theme of this powerful work of science fiction is what is the value of a life and how do we determine what is human and what isn't. The reader is presented with a world where there is apparently a very clear delineation between replicants or robots and humans, but as the novel continues, Dekkard comes to realise more and more that there is no such delineation and the boundary line between what marks somebody as being human and what indicates that they are nothing but a replicant becomes more and more blurred. Perhaps one of the most powerful lines in the whole novel is the following quote:
Do androids dream? Rick asked himself.
This reflects the confusion and conflict that Dekkard suffers. Roy Baty seems to have just as many dreams as Dekkard himself does, yet Dekkard is sent to kill Roy as if he were not human or his life doesn't matter. This quote picks up the philosophical musings of the title and is used to explore the central question of the novel, which focuses on the meaning of life and what determines that we are human.