What does it mean to say that the fall affected reason?
The problem that plagues the reasoning of human beings is the problem that radically affects every human heart. It is the problem of sin. Because of our sin, and our slavery to our sin, we seek to oppose God and His revelation. This opposition is no minor issue. It is an opposition that runs through every faculty of every human being. So radical is this opposition that Scripture declares us to be "dead in our trespasses and sins," (Ephesians 2:1). This "death" means, in part, that in our thinking, in our emotions, in our choices, we inevitably seek to deny God His proper place, and we inevitably make choices that will satisfy us, in the first place. Because of sin, our minds are hostile toward God; we refuse to submit our thoughts to Him. We continually think and act as if we are not His creatures, accountable to Him. Therefore, all of our thinking is substantially skewed.
One man has used the analogy of a buzz saw. The saw may be expertly sharpened and it may cut with precision. But the saw is set at the wrong angle. Everything it expertly cuts will not fit its intended target; it always cuts with the wrong slant. So also with our reasoning capacity. We can still reason and think, sometimes as "experts" in our field. But even such expert reasoning, if it fails to recognize God and His creating and sustaining activity, as well as our creaturely obligations to submit to Him in all things, will not "fit" the way the world actually is.
K. Scott Oliphint, Ph.D. is professor of Apologetics and Systematic Theology at Westminster Theological Seminary.
For Professor Oliphint's full treatment of this question:
K. Scott Oliphint, Covenental Apologetics: Principles and Practice in Defense of Our Faith.
Professor Oliphint's recommended extra resource on this question:
Cornelius Van Til, The Defense of the Faith.