the full text of C.S. Lewis's The Abolition of Man can be found online (also here as a pdf and for Kindle). This short book (61 pages) is an argument for objective moral law — what has been called Natural Law and Lewis called the Tao. From Wikipedia's description of the book:
The Abolition of Man is a 1943 book by C.S. Lewis. It is subtitled "Reflections on education with special reference to the teaching of English in the upper forms of schools," and uses that as a starting point for a defense of objective value and natural law, and a warning of the consequences of doing away with or "debunking" those things. It defends science as something worth pursuing but criticizes using it to debunk values—the value of science itself being among them—or defining it to exclude such values. The book was first delivered as a series of three evening lectures at King's College, Newcastle, part of the University of Durham, as the Riddell Memorial Lectures on February 24–26, 1943.
From the second lecture (the second chapter of the book) titled "The Way":
.... This thing which I have called for convenience the Tao, and which others may call Natural Law or Traditional Morality or the First Principles of Practical Reason or the First Platitudes, is not one among a series of possible systems of value. It is the sole source of all value judgements. If it is rejected, all value is rejected. If any value is retained, it is retained. The effort to refute it and raise a new system of value in its place is self-contradictory. There never has been, and never will be, a radically new judgement of value in the history of the world. What purport to be new systems or (as they now call them) 'ideologies,' all consist of fragments from the Tao itself, arbitrarily wrenched from their context in the whole and then swollen to madness in their isolation, yet still owing to the Tao and to it alone such validity as they possess. If my duty to my parents is a superstition, then so is my duty to posterity. If justice is a superstition, then so is my duty to my country or my race. If the pursuit of scientific knowledge is a real value, then so is conjugal fidelity. The rebellion of new ideologies against the Tao is a rebellion of the branches against the tree: if the rebels could succeed they would find that they had destroyed themselves. The human mind has no more power of inventing a new value than of imagining a new primary colour, or, indeed, of creating a new sun and a new sky for it to move in. .... (Abolition of Man, Macmillan, 1949, pp. 28-29)Full text of The Abolition of Man