Actually what Cogito. . . er, what I said, paraphrased, is this:
"This sentence is indisputable proof that that which is responsible for this sentence being here exists."
. . . and it is.
Your senses, assuming you can read that sentence, provide you with indisputable proof that that sentence exists.
Now the question becomes, how did the sentence get on your monitor?
This is an entirely different question and one that requires an inference from experience to find the most probable explanation.
Given sntjohnny's epistemology, the sentence quoted above is as likely as not to be the result of tiny invisible elves busily at work inside your computer.
His "theory" might run like this: Computers are not connected to the outside world at all. They are self-contained. What is believed by some to be communications coming into their computers from other people is actually only the work of little elves who live inside computers.
Rational folks, however, will not find sntjohnny's belief convincing. They are more likely to believe that the quoted sentence most likely was constructed by a human author who has access to the internet. This is an imminently more reasonable inference to draw from the facts of the situation as we know them and from our experience on earth.
To recap: From our observation of the sentence, "This sentence is indisputable proof that that which is responsible for this sentence being here exists," we know that the sentence exists because it appears on our monitors.
From there, based on our experience, we can easily infer the phenomenon most likely to be responsible for that sentence's appearance and for all the others attributed to "Cogito."
It could be chance, it could be a computer glitch, it could be tiny elves, it could be gods, it could be extraterrestrials, it could be a computer program, etc.
But, based on experience, it is virtually indisputable that a human being with access to the internet is responsible for the messages on this board that are attributed to "Cogito." [Given the progress made in AI in recent years, this inference may not be nearly as strong in the future as it is today.]
Snjohnny and people who share his epistemology see no essential difference between possibility and probability. For them, the phrase "Well, it's possible, isn't it?" has real significance. In their mysterious world, it is as likely that an event happens due to the work of invisible elves and gods simply because "Well, it's possible, isn't it?" as it is that something more natural and much, much more probable is responsible for it.