Self-Evident Truths Vs. Self-Defeating Lies

You know what really churns my butter? People who challenge truths we hold to be self-evident. One of these recent challenges (which really isn't so recent, so much as it is prevalent today) is against knowledge. We can't really know anything for sure. Truth is what we make it to be.

Okay. But if that's the case, how can we know that we can't know anything for sure for sure? (i.e. how can we know for sure that we can't know anything for sure?) How can we know if that is true? Is the statement "truth is what we make it" true or false? It seems it matters no longer, because it has become something that it isn't.

That is, these challenges have become lies we hold to be self-defeating.