Some people argue that we are very complex "machines". From this starting point, you could argue that someday we will be able to invent "machines" that are intelligent and conscious (eg. AI) just like we are. This makes many people uncomfortable, and those people claim that you could never make such a "machine". In those cases, an interesting question to ask is, "How do you know that you are not a machine?". The following is written by Marvin Minsky, in an afterword to Vernor Vinge's novella "True Names":
"Ridiculous," most people say, at first: "I certainly don't feel like a machine!".
But what makes us so sure of that? How could one claim to know how something feels, until one has experienced it? Consider that either you are a machine or you're not. Then, if, as you say, you aren't a machine, you are scarcely in any position of authority to say how it feels to be a machine.