In flat space-time, an accelerated observer sees a horizon behind him. But any inertial observer sees the events behind that horizon. Around a black hole, a distant observer sees a horizon, a freely falling observer sees none. In all these cases, one observer can say "there is nothing behind the horizon" and the other says "of course there is something, I can see it". Of course, the one who sees behind the horizon, in both cases, cannot tell the other what he is seeing. Does this mean that it is correct to say that "nothing is behind a horizon"? Or is it more correct that there is something behind it?