Attack on the idea of universal truth and the values that derive from it is stronger now than in 1944 when this book was written. Yet, despite many rhetorical defenses of universal truth and values launched in the "culture wars," this remains one of, if not the best, defenses of universal truth and values.
Lewis believes in Natural Laws - laws of morality, such as duty to children, parents, elders, the "golden rule," mercy, magnanimity, justice - which have been accepted both throughout history and by varied cultures. Lewis calls these laws "the Tao."
The problem as Lewis outlines it, is that if nothing is self-evident (i.e., true), then nothing can be proven. And, if nothing is obligatory because it is self-evident, then nothing is obligatory for its own sake, i.e., because it is true. If nothing is obligatory, then rules of conduct are subject to pleasure or whim and are enforced only by power of some over others. Ultimately, this robs of us our humanity. Lewis says, "A dogmatic belief in objective value is necessary to the very idea of a rule which is not tyranny or an obedience which is not slavery."
The consequence of rejecting the idea of universal truth, or "the Tao," is the destruction of the society which rejects it. This is, as Lewis says, tragically comical because "we continue to clamour for those very qualities we are rendering impossible."