I don't know why it is, or even that it is. I made no such claim. What I said was that "Thought happens" is undeniable whereas "I think" can be doubted, or at least it is more easily misunderstood.
OK, I think I got it.
The claim that "I exist", in the form of "I am", is the conclusion of the Cogito and not the premise. The premise is simply "I think". We must assume the premise is true in order to accept the truth of the conclusion.
Descartes was actually more rigorous than that. His basic premise was "I can doubt everything except the fact that I doubt", if I remember it correctly. He realized that self-consistency is key to truth. Then he forgot about it...
when I cogitate on the Cogito, even though I might be thinking about words, those words do not have to be defined to anyone else's satisfaction.
Actually they do, at least if you are interested in true insights. It's not a coincidence that the people who are best at discovering truths are also best at expressing them. It's really the same thing.
Of course a person can always amuse himself with flights of fancy. Nothing wrong with that. I just think truth is more fun.
using the terminology I use internally in that way, I know thought happens. I can choose other words to identify the things I call 'thoughts', and I can choose other words to refer to changes in time which I called 'happening', but the experience of thoughts occurring and changing is undeniable by me.
I could perhaps argue that the moment you stop to ponder that "thought happens", you are no longer thinking. So all you have is a recollection of something that might have happened but no longer does. If you think Descartes is no longer thinking, then perhaps you aren't either?
going into this, I made a big assumption about the identity of the "I". How can I be justified in assuming that just because I am in command of this particular keyboard, I can claim to be the thinker? I can't. And so, that introduces doubt which makes "I think" more doubtful than "Thought happens". And, going beyond that doubt to draw the inference that because "I think" therefore "I exist" adds even more doubt. First because we can't be sure the inference is valid, and second because we have introduced a new term, 'existence', which brings with it another whole group of questions.
So you think there are things you cannot doubt, other than the fact that you can doubt anything? That doesn't seem consistent.
I hope that clears up at least some of the confusion.
I hope so too.