I think; therefore, I am

I see it sometimes said on the internet that the only thing we can know is that we exist, and this is based on the cogito. But if you think about it, one would have to know a handful of things in order to justify belief in one's own existence. The cogito, after all, is an argument. The conclusion is that "I am," but that conclusion is drawn from premises. The argument goes something like this:

1. If I think, then I exist.
2. I think.
3. Therefore, I exist.

Or, one could begin with the premise that, "If I did not exist, then I would not think," which is logically equivalent to the first premise. One could just as well begin with the premise that "All thinking things exist" or "Whatever thinks, exists." These variations amount to the same thing.

Since that is basically the line of reasoning, there are three things one must first know before drawing the conclusion that they exist. They would have to know the truth of the first premise, the truth of the second premise, and that the conclusion follows from the two premises.

If one knows that they exist based on the cogito, then they ought to admit that their existence isn't the only thing they know. They also know three other things.