Are they not real because they are not pure?.
You would constitute them as real if you saw them walking down the street because you would rely on your physical senses to be true. You do the same for the physical world you walk down the street assuming everything is real just by physical features. Just like Nagel argues how do you know you can rely on physical features to be accurate? .
Your really can't you would be arguing in circles as Nagel stated. You could argue that the replicants really exist as humans because they have the physical and characteristics of a human. What about the fact that they aren't pure? These replicants don't come naturally like a human really does. So you can make valid arguments that they exist as a human or that they aren't real. .
Same can be said for the physical world. There is no concrete knowledge of the existence of a physical world. All anybody can rely on are your senses. Everything as it seems has entered through your mind. You cannot be sure if everything is as it seems just like the blade runner cannot be sure if that person he met was a replicant. He can't depend on how things seems so you should not either. .
Since you can't rely on everything you need to always search for knowledge. This is where epistemology comes in. Epistemology is the constant search of knowledge. It is the constant questioning of what we know. It comes from the Greek term Epistem, or knowledge. .
Blade runner is constantly searching for knowledge looking for the replicants that blend in the human population. He has to always be skeptical about the situation and about everyone he meets. He doesn't always assume that everything is as it seems. .
This works in philosophy as well. You don't know if there is a physical world outside your mind. You don't know if there is anything outside your mind because everything you know is obtained through your mind. So how do you know anything exist out side your mind? There my or may not be an external world as stated by Nagel.