Some will object, "But dont we know that we are in this room, or that 2 plus 2 equals four, or that grass is green?" To answer that objection, we must define the words "know" and "knowledge."
There are three sorts of cognitive states: knowledge, opinion, and ignorance. Ignorance is simply the lack of ideas. Complete ignorance is the state of mind that empiricists say we are born with: We are all born with blank minds, tabula rasa, to use John Lockes phrase. (Incidentally, a tabula rasa mind a blank mind is an impossibility. A consciousness conscious of nothing is a contradiction in terms. Empiricism rests on a contradiction.) At the other extreme from ignorance is knowledge. Knowledge is not simply possessing thoughts or ideas, as some think. Knowledge is possessing true ideas and knowing them to be true. Knowledge is, by definition, knowledge of the truth. We do not say that a person "knows" that 2 plus 2 is 5. We may say he thinks it, but he does not know it. It would be better to say that he opines it.
Now, most of what we colloquially call knowledge is actually opinion: We "know" that we are in Pennsylvania; we "know" that Clinton either Bill or Hillary is President of the United States, and so forth. Opinions can be true or false; we just dont know which. History, except for revealed history, is opinion. Science is opinion. Archaeology is opinion. John Calvin said, "I call that knowledge, not what is innate in man, nor what is by diligence acquired, but what is revealed to us in the Law and the Prophets." Knowledge is true opinion with an account of its truth.
It may very well be that William Clinton is President of the United States, but I do not know how to prove it, nor, I suspect, do you. In truth, I do not know that he is President, I opine it. I can, however, prove that Jesus Christ rose from the dead. That information is revealed to me, not by the dubious daily newspaper or the evening news, but by the infallible Word of God. The resurrection of Christ is deduced by good and necessary consequence from the axiom of revelation.