The problem isn’t that it might be possible for someone to think without existing, or to experience things without experience existing. The problem is that assigning a probability of 1 to something means “this way of knowing is such that it cannot fail.” And just because you have to exist in order to think, without fail, does not mean that the “way of knowing” which is involved is a kind which cannot fail. In other words, 13 cannot fail to be prime, but the way we know that “13 is prime” is a way that in principle can fail. “I exist” is no different. If you think that, you exist without fail, but not by reason of your way of knowing. So you cannot attribute infinite certainty to it.
The problem isn’t that it might be possible for someone to think without existing, or to experience things without experience existing. The problem is that assigning a probability of 1 to something means “this way of knowing is such that it cannot fail.” And just because you have to exist in order to think, without fail, does not mean that the “way of knowing” which is involved is a kind which cannot fail. In other words, 13 cannot fail to be prime, but the way we know that “13 is prime” is a way that in principle can fail. “I exist” is no different. If you think that, you exist without fail, but not by reason of your way of knowing. So you cannot attribute infinite certainty to it.