Cogito ergo sum? We will aks ourselves, wether this scentence is true or not. Techincally, the question is stupid because assumptions base on words, wich were defined before. So every sentence could be true as long as the defintitions are fitting. So what we really should be trying to do is define our words. But in this case there is no true or false, definitions have to give the word a meaning, that is usefull for our life. Finding the word bed for a thing you sleep in is usefull to describe the use of an object, for example.
So lets agree on the definitions of the words in the statement "I think, therefore I am": The way we think of the words thinking and being, we need some kind of perception for that. At least those are manmade words and therefore require imaginations based on our perception. So yes, if we follow our concepts of those words, the statement would conform with our concept of truth.
But is there any reason, why all of those concepts whould be "universally true" and not just a bunch of definitions compatible to our undersanding of logic? Descartes clearly assumes this "universal" level of exisence, standing above our perception (he wants to get away from this fallacious level of perception and therefore proving his "absolute" existence). But with the knolege, that all of our words and true- or false-assignments just happen inside our perception and thinking, Descartes getting away from those particular levels was not successful. Following the same principle I could say: "I see, feel an taste the table, therefore it exists". The usual response is that my senses could be wrong. For example an optical illusion. But how could we decide, wether something exists if not using our perception? Why would the word be used then?
Things exist, when the majority of people precieve it. And when we talk about something as an illusion, we men that our perceptions are not copadible with the preceptions we would assume to have based on the laws of physics. But those laws are also just attempts to predict our perceptions, becasue they follow a certain rule sometimes. For exaple, we invented gravity because we saw things falling to the ground. And if perceptions cannot be explained by those laws we improve them. Apparently, our understanding of existence (or the relation of perception and science) does not need something absolute, things are true or exist, when it makes sense for humanity.
When people were assuming that stars were holes in the sky, this was true. Our current theory just fits better to our perceptions and because of that we refer to them as true. In this way we can improve this breathtakingly complex perdicting-machine, called Physics. But it will still not lead us to an absolute truth. A concept of this truth standing above our perception is simply not usefull. And that is what it really is about. We think and therefore we are? Yes, because we said so.

Btw, this is my first article. I am not a native speaker and this article is certanly not easy to read. I just wanted to write my thoughts down and share it somewhere...