J.P. Moreland On Skepticism and Knowledge

[I]f the skeptic doesn't offer a reason for his skepticism (and just keeps asking "How do you know?" each time the particularist makes a knowledge claim), his skepticism can be ignored because it is not a substantive position or argument. If, however, his skepticism is the result of an argument, then this argument must be reasonable before it can be held as a serious objection against knowledge. However, if we did not know some things, we could not reasonably doubt anything (e.g., the reason for doubting my senses now is my justified belief that they have, or at least may have, misled me in the past). Unbridled skepticism is not a rationally defensible position, and it cannot be rationally asserted and defended without presupposing knowledge.

Kingdom Triangle, p. 125