You can never know anything for sure.

Think about that for a while....

This may help:

Let's Start with Breakfast...
Imagine the following comical scenario. A fellow Earthman runs up to you, with glazed and feverish eyes, and proceeds to explain how he has discovered a fundamental and absolute truth about himself. When you ask him to tell you this awesome truth, he blurts out this: "The fundamental truth is that I cannot pronounce or write the word 'breakfast'!" You are not sure you heard him correctly, and so you ask him to write the truth down on a sheet of paper. He then writes legibly on the sheet: "I cannot pronounce or write the word 'breakfast'."

There is something obviously wrong here (other than the fact that the guy's elevator doesn't go all the way to the top floor!) and what the obvious wrong is is clear--the speaker contradicted himself in the process of speaking. He rendered his 'truth' ineffective--he stultified himself.
This is a special case of reductio ad absurdum -- but the absurdity was that "what he said" (the words, pronunciation, the speech act itself) contradicted the "what he said" (the content, the intention, the meaning).

Now let's generalize this type of argument.

The Nature of Self-Stultifying Statements
A self-stultifying statement is a statement that contradicts:

1. itself;
2. the case it advances as proof (if any);
3. the presuppositions inherent in the subject matter being discussed;
4. the presuppositions inherent in the speech act.

Let's illustrate these cases with a simple example.

Case 1: Contradicting itself ("Even though a horse is black, it is not black.")
Case 2: Contradicting the proof ("This black horse is not black")
Case 3: Contradicting the subject matter ("This horse is black half of the time"--horses don't change color often.)
Case 4: Contradicting the speech act ("I am a black horse"--semantic acts, of the English variety at least, are not performed by horses.)

If you want to read more on this, it can be found here