Often, in the course of my collegiate education, other persons in class (often the professor) will say something to the effect of "Well, we can't know what truth is for sure" or "What's true for x is not necessarily true for y." This attitude is so prevalent throughout academia that it takes a great deal of sorting to snap out of it and dissect it. Gradually, I've awoken to this attitude by the help of others - most notably by reading the writings our current pope, who sees this attitude as cultural suicide for the West (more on that some other time).
The logical fallacy that each of the above statement falls to is so thinly veiled that it's almost naked, yet we are so used to it that we are conditioned not to see it. The purpose of any intellectual discussion is to uncover the truth about something. In a court case, we swear to tell the truth so that justice may be administered properly. When we obscure truth, we thus distort justice. When we lie, we obscure truth.
So, when someone says "We cannot know truth for certain" we must assume that this person is telling the truth about their beliefs, and that they know that their statement is certainly true. See the error in logic here? If I say "the sky is blue," I genuinely mean that the sky is blue, and I genuinely wish to convey this truth to you. I would not claim that the sky were blue if I knew, deep down, that it was red - it would be dishonest. Thus, when someone presents the unknowability of truth, we must assume that they do not know if what they are saying is true. This is simply the extension of the logic presented.
The second statement is similar to the first. If truth is not the same between x and y, then one can claim that my truth is better than the truth of that statement. Again, the person arguing in this way is caught in an intellectual pickle.
Long story short, when someone says "Truth cannot be known for sure" ask him or her if that statement is surely true. I would be interested to hear the ensuing discussion.
