This is an excellent passage from The Abolition of Man
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 judgments. 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 has never been, and never will be, a radically new judgment 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 color, or, indeed, of creating a new sun and a new sky for it to move in.
-C.S. Lewis, The Abolition of Man, p.56
Modernism consists mostly of new systems of value that appear different because they are “sollen to madness in their isolation”. Yet they only have any validity because they still appeal the natural law, it least some of it.
Post-modernism is the ridiculous attempt to imagine a new primary color. To contradict itself by using rules about what is valuable/not valuable or good/bad to undermine the definition of good and bad. This can only fail and it only ever does.