I very well understand the thing about how what I see as blue, you may see as something else, but we call it the same thing and think they're the same. But the rest of this whole thing is real confusing...
There is a man named Descartes (DAY-CART) who wanted to see if it was possible to doubt the existence of everything, he went about it like this.
Descartes's first wave of doubtHe started with our senses, it is common knowledge that sometimes our senses deceive us, when you put a pencil into water it appears to bend when it actually hasn't changed shape and from a far distance you may see a tower that looks round but up close you find that it is square. From this he stated that since our senses deceive us it is logically possible that the world around us could all be a deception.
However it was pointed out to him that in ideal conditions there is no reason for us to believe that our senses are always wrong so he moved on to his second wave.
Descartes's second wave of doubtHe then thought about the world of dreams and how during a dream it is very hard to tell that it is a dream, from this he said that the world around us may be nothing more than a dream.
However, dreams are like paintings in that they reflect the truth, for example two and two will make four in a dream just as well as they make four in the waking world so with this wave it was impossible to doubt everything, which brought him to his third wave.
Descartes's third wave of doubtEverything around us is a trick from evil demons (don't laugh). There are demons around us which control everything, when you look at a chair it is the evil demon which makes you think that you are looking at a chair, when you add two and two to make four it is the evil demon tricking your mind into thinking that it makes four when possibly in reality it makes five, when you talk to someone it is the evil demon tricking you into thinking that someone is there to talk to,. It is the evil demon which deceives you into believing there is a world around you when in fact there is nothing around you.
With this idea, Descartes was able to doubt the existence of the world around him and everyone he met, even that he has a body (evil demon tricking him in to believing that he has a body). However, he couldn't doubt the existence of his mind. In order for the evil demon to deceive his mind, there had to be a mind that could be deceived and from this he decided that all he could prove he was, was a mind, a thinking thing. In other words "I think, therefore I am".