Why does the fact that it is possible to make mistakes mean that sense perception is deceptive? Why should we expect it to be infallible? People are not infallible. An auto is not unreliable because it might break down. An auto is unreliable if it breaks down a lot. Should we expect that an automobile will never break down? No more than we should expect that sense perception will never lead us to error.
I know if I am not mistaken. That I might be mistaken does not show I do not know. I know that Saturday is the 26th. That I might be mistaken is no reason to think I don't know that Saturday is the 26th. To show I don't know that Saturday is the 26th you need to show me that I am mistaken in believing that Saturday is the 26th.
Well, I think what Descartes wanted to say was that, a deceptive sensation could be
the cause of many errors which we perceived as true; but he does not claim that a deceptive sensation is the cause of all mistakes.
I think that, it is necessary for us to diverty our attention from the topic of "rightness/wrongness" to the topic of "certainty/uncertainty."
What Descartes had intended was not the former pair, but the latter. Therefore, the right expression of his thought would be: nothing can be called right and certain unless it is clearly and distinctly perceived. But, it is self-evident, as you also acknowledged that, we never wish ourselves to be infallible, for we are by nature peccable and imperfect. Therefore, since we know we are imperfect, then according to D's Principle of Sufficient Reality and P of Adequate Reality, we must be caused by something that is perfect, which is also a self-caused thinking thing. (God as a causa sui)
Finally, in case that you still insist that you can know that you are mistaken, I shall refer to Plato's allegory of cave. In this cave, all men are fettered by ropes from their childhoods, and forced to face a wall on which there are many shadows on it caused by some "real people and some puppets controled by these people", and the fire behind these people. They could not even turn back and see what was really happening. As a direct result, they thought that what they saw directly, that is to say, things perceived by theri eyesight (sense perception) were true.
Sometimes, we are just like these pathetic people, who think they know the truth, but who really do not.
---------- Post added 09-24-2009 at 08:57 PM ----------
p.s. I did mean that we can never know that we are mistaken. I simply meant that we can not always know it.
By the way, personally, I am an atheist. I just retell what I have learned from Descartes.