Francis Schaeffer once described moral relativists as those “who have both feet firmly planted in mid-air.” (See Koukl and Beckwith’s helpful book built on that title.)
An even more vivid illustration is that of Cornelius Van Til who sought to describe the impossibility of unbelieving reasoning if their worldview is employed consistently:
Suppose we think of a man made of water in an infinitely extended and bottomless ocean of water.
Desiring to get out of water, he makes a ladder of water.
He sets this ladder upon the water and against the water and then attempts to climb out of the water.
So hopeless and senseless a picture must be drawn of the natural man’s methodology based as it is upon the assumption that time or chance is ultimate. On his assumption his own rationality is a product of chance. On his assumption even the laws of logic which he employs are products of chance. The rationality and purpose that he may be searching for are still bound to be products of chance.
—Cornelius Van Til, The Defense of the Faith (P&R, 1972), p. 102. (The link is to the newer edition edited by K. Scott Oliphint, but the page number is from the original edition.)
For a similar argument from C.S. Lewis, see Victor Reppert’s C.S. Lewis’s Dangerous Idea: In Defense of the Argument from Reason (IVP, 2003).