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The reason why our sense perception is deceptive can also be explained in the following way: from the fact that there is no demarcation between an awake and a sleeping condition, it follows that what we deem to be true might be something in our dream, but not something that is perceived in reality.
Therefore, the only thing a person is capable of knowing, according to Descartes is, the mode of thought. Simply speaking, I know that "I affirm, I doubt, I believe..." But, whether the thing to which I believe, affirm and doubt is true or not is not certain before God's existence is proved.
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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.