On occasion you might run into the question. "How do you really know?" And then if the question persists, it can turn into a question of whether you know what is real and what isn't. Even to the point of if you can trust your own senses.

Consider that line of thought Further down the rabbit hole. How do you know if you are real? Not a simulation in a virtual reality simular to the movie "The Matrix," or background person in someone else's dream, soon to disappear when they wake up. What if you aren't real?

There are several ways to reaffirm that you actually exist, but one argument stems from the philosopher René Descartes, who said that if you can think then you exist. He coined the phrase "I think therefore I am" to describe that argument. If this works for you that by thinking you prove that you exist, then let's move on. Is there any way to prove that you are not real?

Consider this line of thought on someone who has already died. Could be a relative, or could be a famous celebrity. How do you know they are real? Or even were real? When it comes to charished celebrities often a good quote will be credited to them, regardless if they said it or not. The question comes to then if they are who you think they are.

So here is the strange thought.

If you can think, you exist. Therefore you are real. But if your mostly known for something you haven't done, or haven't said, then you are basically a lie and not real.

Dear Not_Now.Soon,

One can doubt one's qualities, but one cannot honestly doubt oneself.

How can you tell that something is real? It never changes!
(change can only occur against a background, but that implies that the background is more real than that which changes over it)

Your qualities are changeable, thus not truly real.

You, on the other hand, are always you, were always you and will always remain you - thus you are as real as real can be!

«If you can think, you exist.»

First note that thinking is not a requisite for existence.

More importantly, your claim to exist depends on the realness of existence - if existence itself is an illusion, while (as above) you are not an illusion, then you cannot exist. Yes your qualities exist even if existence itself is an illusion, but these are YOUR current qualities, rather than you.

«But if your mostly known for something you haven't done, or haven't said, then you are basically a lie and not real.»

What others think of you is a transient quality, which doesn't say anything about you.

«The question comes to then if they are who you think they are.»

That's a very easy question: Who you think they are is a thought (of yours), yet they (whom you consider to be dead) are not a thought - thus the answer is 'No'.

Not_Now.Soon,

What you touch on, for a moment there in your second paragraph, is the problem of hard solipsism: How do I not know that my mind is all that exists? How do I not know that I’m just a brain in a vat?

Apart from the fact that it would be incredibly arrogant of me to believe that my mind came up with all the beautiful pieces of art around that world, and all the brilliant music that has been written over the millennia, the simple answer is that I don’t know.

But my experiences are real enough for me, and my actions have predictable-enough consequences to still want to go on living my life in the best way that I can, and to this extent, I trust my own senses. Whether I’m a brain in a vat, a simulation, or whatever, is apparently of no consequence to me.

<<If you can think, you exist. Therefore you are real. But if [you’re] mostly known for something you haven't done, or haven't said, then you are basically a lie and not real.>>

There is nothing profound or paradoxical here. All you are doing is engaging in equivocation with your use of the word ‘real’. When you first mention being ‘real’, you are referring to the state of actual existence; when you mention being ‘real’ (and a lie) in your concluding thought, you are referring to the state of being artificial (or not genuine).

http://en.oxforddictionaries.com/definition/real

If you want to experience reality borrow money from a loan shark and don't pay it back.
Your reality and his will consequently collide and you will have positive proof that you exist, at least in his imagination.

Rene Descartes stated - "I think, therefore I am."

I suppose what he was referring to was the fact that
your thinking results from your exitence. So if
you didn't exist you wouldn't be able to think in
the first place.

Does that mean that some humans may not exist?

Wow!

To Yuyutsu,

[One can doubt one's qualities, but one cannot honestly doubt oneself.]

Oh I would say other wise. If there is a difference between your body and your soul,ma person might think the body is the shell but the soul is who they are. From the stand point that the soul out lives the body, this makes sense, because "you" are the parts of you that last. However, if a person has a head injury, and 1) doesn't remember who they were like before the injury; as well as 2) is described as as different person before the injury verses after the injury. That person might at one time question if they are the same person (the same soul) or if the pervious person died and left, and instead you remained.

I doubt this is the only instance of doubting one's self, but this is a quesstion I held some time ago when I was younger. I'd assume other doubting of yourself existing or not might also happen if a person has hallucations, or multipal personalities.

[thinking is not a requisite for existence.]

I agree, but by being able to think, that proves that you exist. I think that is what René Descartes was getting at.

[What others think of you is a transient quality, which doesn't say anything about you.]

But in the terms of what is real and what isn't, misconceptions are a big deal. If we take a step back away from knowing you exist, to instead knowing that a person loves you, or hates you. That is in the area that can be equally hard to prove, but often people can say "I just know." Misconceptions and astute judgements might say more then you'd expect. Some people strive to live up to the expectations they are given,many others strive to be free of qualities they are seen with. That's not counting how we view others and they view us affects the interpersonal relationship of the people. People can build a person up in their head, then find out how wrong they were.