On SparkNotes, Rene Descartes apparently knows some things to be true as long as he is "attending" to them.
These clear and distinct perceptions are only indubitable so long as he is attending to them. As soon as they fall out of awareness, the doubt can creep back in. Once again, he can begin to wonder whether it was an evil demon who caused him to believe in the certainty of these truths. Suddenly, things do not look too rosy for his system of certain knowledge; if he needs to keep every truth perpetually before his mind, then he cannot expect too make much headway in unraveling the facts of nature.
From what I gathered, he only knows something is true as long as he is thinking currently thinking about it. But how does he know that? Since thinking about something takes time, and does not happen instantly, why can't he doubt what he was just thinking?
Apparently, Descartes tries to solve this by introducing God. But thinking, and actually proving, the existence of God could also be subject to his "evil demon".
To rephrase the question: When trying to think logically about a topic, it always takes time. It cannot happen instantly. How do you know, at any given point, that the rest of what you thought is true?