Nothing can be known for certain. There is no way to establish, incontrovertibly, the truth of any proposition (including this one). All your knowledge is in the end the result of argument, thought (hopefully rational), and a subjective judgment made by you.
When people realize that knowledge cannot be shown to be objective Truth (with a large T) they often flip over to the opposite extreme of relativism. They say that all knowledge is equally subjective and prone to error, so any answer you hold is as good as any other answer (for example, the answers in a textbook or the opinions of an expert). Similarly, people engage in moral relativism, e.g. “Hitler was only following his own moral code and I cannot say that he was wrong.”
Relativism is based on a truth, but both the forms I described above are utter balderdash, and I will explain why.
Firstly, all knowledge is not equally subjective or unreliable. Perhaps I claim that the moon is made of green cheese. Well, that’s so ridiculous a theory that you can basically reject it out of hand. The moon isn’t green and it doesn’t look a lot like cheese. Men have been to the moon and it was rock. They brought back pieces and they were rock. The moon is most definitely made of rock. You can’t prove that the Truth isn’t that the moon is made of green cheese and all the other stuff is just lies and errors, but it stretches credibility and reason far beyond the breaking point to suggest such a thing. It may be impossible to demonstrate the absolute certain truth of a statement, but statements can be demonstrated false, and one can be relatively certain about the truth of a statement.
Part of knowledge is rational judgement. You have to decide for yourself what makes sense. The moon being made of green cheese does not make sense. A moon made of rock does make sense. You use your good judgement and consider the evidence of your senses. If you are rational you try to consider all the possibilities, and keep an open mind. However, obviously some ideas make sense, and some do not. All ideas are not equal, even if we can never be certain which ideas are True.
Thus it is ridiculous to ignore propositions because they might not be true. All propositions should be evaluated and rejected or accepted on the relative merits, rationally judged. Most often people use relativism as an excuse to hold irrational, or at least poorly supported opinions as being “just as good” as more widely accepted explanations for the same phenomena. However, opinions are not “just as good” simply by virtue of being possible. Argument and evidence must support them. To be accepted as the best possible explanation a proposition must be clearly better than all the other available explanations by virtue of its supporting arguments and evidence. The fact that a proposition is widely held is an indication (though not a guarantee) that it is the best possible explanation. On the other hand a proposition regarded by most people as silly or wrong might well be both silly and wrong.
The abuses of moral relativism are best represented by the “I cannot say Hitler was wrong” example above. This is at best a horrible misunderstanding of the truths of relativism, and at worst an abdication of personal responsibility. As I said above, relativism does not make all positions equal. It also does not mean that you cannot hold an opinion or proposition to be correct. The difference once you have accepted the realities of relativism is that you have to be prepared to argue for that proposition in competition with other propositions. Consider then the arguments Hitler could make for his actions. Personally I cannot think of any argument which could even begin to convince me that Hitler was acting morally. He may have been acting according to his own principles, but those principles were wrong. I have never heard a convincing argument for holding those principles and I doubt I ever will.
So, Hitler was wrong (though wrong is too mild a word). The difference with relativism is that he was wrong not because of some absolute set of rules we know are correct, but because his propositions do not survive in competition with other propositions. We cannot be sure that the position we hold now is an absolute truth, but we can be fairly certain it is better than Hitler’s version.
The truth of relativism is that we must judge all propositions on their merits, and even if we choose an answer that choice can never be final. We must always be prepared for new arguments and new propositions. We can never be sure that we have a final, complete, absolute Truth in hand.
The trap of relativism is that you might think it makes it impossible to decide that a proposition is true and that others are false. This is not true. A proposition can be regarded as true as long as you can make a convincing argument that it is better (it makes more sense and agrees better with the evidence) than any other available proposition on the same topic. Propositions can easily be argued to be false, by showing they are self-contradictory, or contradict available evidence for example. Once you have found, by a series of arguments, the best available proposition then it only makes sense to accept that proposition as true. At the very least you have been convinced by argument that it is the closest thing we have available to Truth. It makes absolutely no sense to ignore those arguments, and the only alternatives are choosing a proposition which is demonstrably less convincing, or saying there is no answer at all (which is both philosophically unsatisfying and in many cases practically impossible as well as being useless). In the face of uncertainty we can still be relatively certain.