No, really? How do you know? You may think you exist but you can’t really prove it because your existence may be the figment of someone else’s imagination. This universe could easily be something similar to the Matrix in the films of the same name.
As such you might not even be reading this blog at all. You might be in some booth underground dreaming that you are reading these words when you are actually unconscious. Or perhaps you are in a coma and the reality in which you think you find yourself is merely the last embers of your brain giving you a tenuous existence before you kick the bucket.
By that same logic, you don’t know if other people exist either. Are they real or figments of YOUR imagination? You see people all the time but you have no idea if they are thinking or are capable of sentient thought. You just assume so because they appear to be sentient. They could easily be created solely to deceive you into thinking that you are not the only real person in existence. You might very well be the only person who is truly alive, you can’t prove otherwise, can you? Who is to say that you are not really wired up into a massive computer which feeds you mental images of a real life that is not in any way like the life you really lead?
No wonder some people are paranoid. But do they have a right to be paranoid over such things that should be taken for granted? Maybe, who knows?