You have to start at the beginning.
What do you know with certainty? That you are aware of your thoughts.
Since you are aware of your thoughts, you can conclude that you exist.
Now we have "Cogito ergo sum".
Good... you exist... next step.
What do you know about your thoughts with certainty? Only that they exist and that you are aware of them. Their nature, and the nature of their content, however, remains unknown.
So it's too early to move on to claiming the existence of a true objective world around you, based on the mere content of your thoughts.
Don't blame me if you don't like the truth. .
But how do you know that you are aware of your thoughts? If you question the content of your thoughts, I see no reason why you shouldn't question your ability to have a thought, and therefore exist. Look at it this way. A thought is a human description of a psychological concept. To know what a thought is, one must accept this external definition. This is where the logic breaks down, in my opinion. If you cannot trust the content of your thoughts, then you cannot understand what a thought is, or the concept of existing, because those are thoughts! Example: you could be programmed by a higher being to say, I think, therefore I am. That's a possibility! In that case, you don't exist, because you are not really thinking, but only following a script. Because it's a possibility, according to that logic, you cannot accept it as truth.
Conclusion: One cannot know ANYTHING for certain, even one's own ability to think as poof of existence. I see two solutions to this paradox.
1. Because one cannot know what is real, what is real is irrelevant. What matters are the perceived, subjective consequences of experience and interaction. Real, or imagined. If one accepts human experience as limited to subjectivity, then objectivity has no meaning, and can be disregarded as moot. Therefore, because pure objective fact has no meaning, as it cannot be interacted with on any level, we can only hope for high probability of universal truth, not certainty. In most applications, this is what the word objective, as used in human language, means.
2. We are using a language which is designed for interpersonal communication. The fact that there is a word objective, means that it does exist, within our frame of reference. Because absolute certainty does not exists in this common reference, our language does not address this issue. Therefore, to state that objectivity cannot exists is ignoring the true definition of the word, rendering this discussion moot.
Now If you respond with something along the lines of, "this is my own personal illusion, this is how I think it is, therefore it is to me," you did not comprehend my post. I am not talking about what is, and what is not, I am talking about our fundamental inability to know anything for certain. Therefore, to try and find any certainties is frivolous. Well then? What are we supposed to do if we can't know anything for sure? I'll give two examples:
1. I have class in one hour, assuming my subjective interpretation of time is the same as that of my professor. I must cross 4 streets to get there, I cannot know for sure if I will be run over, and killed on my way. By the way, what is death really? I have never died, how do I know that will happen if I impact a 4,000lb SUV at 40mph? I also think, but don't know that if I don't attend class, I will fail. What if I was wrong that it's only four blocks, what if it has changed to 100,000,000? What if the world doesn't exist in the first place? What do I do?
I go to class. Why? Because certainty doesn't matter. What matters is trial and error. What matters is probability. What matters is that the universe has been subjectively objective for the 24 years of my existence. This is similar to how all animals live. What works, what doesn't? Not what is real? What isn't. That doesn't matter. What matters is the subjective satisfaction I subjectively feel from thinking that the A I received is still considered the subjective standard of subjective success.
Species which ask themselves, "I see a large predator closing distance very fast. It looks like it want's to eat me for lunch, but how do I know it isn't my imagination, or that my subjective view of threatening body language is different for this particularly large and fast creature?" are selected by nature not to pass on their genetic makeup.
This is why the "fight or flight" aspect of the peripheral nervous system is both necessary, and observed in most all animals. The ability to discuss philosophy over coffee at Starbucks, while nice, is not.
We are given (from God, or nature, or both) the GIFT of a large prefrontal cortex. Let's not offend anyone by using it to say that both it, and the billions of years of existence, are not real. Ironic if you ask me. But hey, you are more than entitled to your belief! That's the beauty of it! However, hypercritically and passive-aggressively forcing your belief on others will get earn you a negative reaction. Refer to Newton's 3rd law of motion, which I believe is applicable here.
Oh yeh! Example 2:
I want to ask a girl out on a date. I cannot be certain she will say yes. I (sometimes

) have the guts to ask. Why? :thumbup:
Hopefully this less rhetorical example will illustrate my point. I'm no philosopher but I'll say this. "I feel bad, therefore I don't repeat. I feel good, therefore I do." To me, that's whats important. If thinking everything is an illusion makes you happy, great. Just don't ever test it (for your sake), or build elevators (for my sake).
One last little experiment: (THIS IS RHETORICAL. DON'T DO IT FOR THE LOVE OF GOD)

may he be praised.
If the vitality of oxygen is imaginary, why not test it by putting a plastic bag over your head? Because the *possible* risks outweigh the benefit of "knowing." If this is your answer, you have just invalidated your claim. If you don't try it, you also invalidate your claim. Because you are using an external, and possibly illusory *probability* that this will hurt you in influencing your actions. Therefore, by the COMMON and INTENDED definition of objective truth, that you decided this would *probably* be ill-fated based on a collection of subjective data (which by definition, is then considered objective) was valid, is an example of what the other posters meant.
Please reply if you disagree!! Seriously, I love a good debate, I'm not being sarcastic (it's so hard to convey tone with text). And everyone here seems very well informed. I also think there is a lot of potential to your proposition, it just seems a little underdeveloped.
For example, I read a theory on how in many equations, T (time) will cancel out. The theory was how time is not a fundamental quantity, but a human construct. There were some serious flaws in his argument, but without wrong ideas, there are no right ideas and inspiration for those right ideas. I think Einstein said something like that, but I'm not gonna try to quote him verbatim. I read through the whole thread and both sides had some great points. This is my point. Sorry if anything came off as a personal attack... I was trying to remain ...omg I'm going to say it...objective. :thumbup:
p.s. How can you explicitly say that what you say is true, when your main argument is that there are no truths other than individual existence? Why waste your time trying the impossible (convince imaginary beings of something that's only true for you, because you cannot know there are others who have the capacity to be convinced.) I didn't follow.