[The intellect of fallen man] may be compared to a buzz-saw that is sharp and shining, ready to cut the boards that come to it. Let us say that a carpenter wishes to cut fifty boards for the purpose of laying the floor of a house. He has marked his boards. He has set his saw. He begins at one end of the mark on the board. But he does not know that his seven-year-old son has tampered with the saw and changed its set. The result is that every board he saws is cut slantwise and thus unusable because too short except at the point where the saw made its first contact with the wood. As long as the set of the saw is not changed, the result will always be the same. So also whenever the teachings of Christianity are presented to the natural man, they will be cut according to the set of sinful human personality.
Van Til, Defense of the Faith, 4th ed., 97
This is a very helpful metaphor for understanding the Reformed idea of the noetic effects of sin. The saw (analogous to our reasoning faculty) does indeed work; it is the same saw that was previously set correctly. Because of its faulty setting, however, even though it works, it always cuts with the wrong slant.
Note by K. Scott Oliphant in Van Til, Defense of the Faith, 4th ed., 97, n. 18.