The one that most clearly sprung to mind for me is Sam Harris, and especially his book The Moral Landscape. Harris is a perfectly distilled example of the naivete and folly that Lewis describes. Harris -- as with many modern men -- wants to use reason so as to see through the foundations of reason, and to appeal to some 'value' that is more fundamental than any of those values that we sentimental humans inherit from our forebears. But, as Lewis says:
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. Therefore never has 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 of 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.
At any rate, a systematic debunking of the naive positivism of Harris is beyond the realm of this post and superfluous. But the manner that Lewis lays bare the conceits of such men -- effortlessly navigating and giving felicitous expression to philosophical ideas about the nature of man -- is impressive. Harris is a potent example of such a man, but he's far from unique as such men are now everywhere occupying positions of influence and power. Lewis was perspicacious on this count.
It isn't the love of reason (to the extent that it's present), or the value of skepticism that Lewis is putting on trial, as he recognizes the great value of both. But Lewis knows that the achilles heal of the rigorous, thorough, devout skeptic is all too often, ironically, his insufficient skepticism:
Their scepticism about values is on the surface: it is for use on other people's values; about the values current in their own set they are not nearly sceptical enough. And this phenomenon is very usual. A great many of those who 'debunk' traditional or (as they would say) 'sentimental' values have in the background values of their own which they believe to be immune from the debunking process.Harris and his ilk don't misstep by appealing to certain First Principles. Indeed, if they wish to reason about morality (or anything else) they can hardly do otherwise, as Lewis clearly sees. It is in denying that they are doing so that they err. But does Harris really believe that there is, or can be, such a thing as a neutral, or assumption-less ground for reason or morality? You would not think such a thing possible for an educated person, but I recently read Harris' short e-book Lying, and a passage in it seems to confirm just that:
Like many of Kant’s philosophical views, his position on lying [that it was wrong in all cases] was not so much argued for as presumed, like a religious precept.Although this is a dubious reading of Kant, the salient point is that the very notion of First Principles appears to scandalize Harris. As if he had reasoned for his moral system at every juncture, all the way down, including at its starting point, rather than presuming that flourishing is desirable and misery is undesirable "like a religious precept." His skepticism is sufficient to question Kant and his First Principles, but apparently such skepticism is inapplicable to Sam Harris and his.