Prerequisite reading: Problem of the Criterion III
I outlined the common sense approach to the problem of the criterion ... an approach called particularism. We start with things we do know and build on them. We don't have to know how we know them or even be able to prove them. If they are fundamental, commen sense types of knowledge, then we assert them and start building our knowledge.
The skeptic does not like this. He knows he can defeat the methodist, and attempts the same attack on the particularist by saying, "well how is that you know what you claim to know?"
Ask him, why should I take your question seriously? What grounds do you have for disputing my knowledge claim?
If he has none, then so be it. His skeptical challenges are marshmellows.
If he attempts to make a real argument, then he has committed a mistake. To make an argument, the skeptic must have some grounds for making the argument. In other words, he ostensibly holds some knowledge that contradicts your knowledge claim. Identify his knowledge claim, and ask him, "well how do you know that?" When he answers, ask "well how do you know that?" Turn the tables. He will get the point. Then buy him a cup of coffee.
Radical skepticism is self-refuting. It presupposes knowledge ... knowledge that knowledge cannot be presupposed ;-) Unbridled skepticism is a self consuming pac man.
An iterative skeptic is the quintessential broken record skeptic ... "says who?" ... "says who?" ..."says who?" ... ad infinitum. The iterative skeptic can be ignored. Not all skeptics are iterative skeptics, however. If you have a serious questioner on your hands, then just ask them "why should I take your question seriously?" That is a very reasonable question.
The skeptic must produce a good case for why your knowledge claim might be false. If he does, fine. You can discuss the specifics and hopefully make some progress. If he does not, then his skepticism-for-skepticism's sake poses no threat.
J.P. Moreland makes a cogent point about particularists and skeptics. From Love Your God With All Your Mind, pg 142
"The particularist and the skeptic have very different approaches to knowledge. For the skeptic, the burden of proof is on the one who claims to know something. If it is logically possible that one might be mistaken, then knowledge is not present because knowledge requires complete 100 percent certainty. Of the two main tasks in the quest for knowledge (obtaining true or justified beliefs and avoiding false or unjustified beliefs), the skeptic elevates the latter and requires that his position be refuted before knowledge can be justified. To refute something is to show that it is wrong. The skeptic thinks avoiding error is better than gaining truth and thinks he must be shown wrong before anyone can claim to know anything.
The particularist elevates the value of gaining as many truths as are available in the world and tries to rebut the skeptic. To rebut something is not to show that it is wrong, but simply to show that the skeptic has not adequately shown that it is true... He places the burden of proof on the skeptic and requires the skeptic to show that his skepticism is true and should be taken seriously before he allows the skeptic to bother him about his knowledge. The particularist does not need to refute the skeptic (show skepticism is false), he merely needs to rebut the skeptic (show that the skeptic has not adequately made his case for skepticism)"
That is the pay off. You don't have to refute skeptics. Just rebut them. Rebutting a skeptic is a more modest goal than refuting one.
Now, some Calvinists raise an entirely different issue about knowledge. They raise the issue of the effect of the fall on man's noetic structures (i.e. his ability to know things). I had one proponent of this view present knowledge as a circle. "A circle", he said, "has no on ramp. There is no way in." In other words, you are either in the circle or you are not. If you are not, then you can't really know reality. You have to be placed in the circle (i.e. by an act of God's grace). Once you are in, then the scales fall off your eyes, as it were. But apart from that, there is no "common sense" way to know anything. It is a fascinating argument ... and out of scope for now :-)