The fatal flaw is the reliance on reason. As Wikipedia puts it,

The Enlightenment included a range of ideas centered on reason as the primary source of knowledge….

Where reason is

the capacity of consciously making sense of things, establishing and verifying facts, applying logic, and adapting or justifying practices, institutions, and beliefs based on new or existing information.

So much of life is — of necessity — conducted in a realm beyond “reason”, where instincts and customs come into play in a universe that is but dimly understood.

By contrast, as the Wikipedia article admits, the Enlightenment — like its contemporary manifestations in pseudo-science (e.g., Malthusianism, Marxism, “climate change”), politics (e.g., “social justice”), and many other endeavors — relies on reductionism, which is

the practice of oversimplifying a complex idea or issue to the point of minimizing or distorting it.

Reason relies on verbalization (or its mathematical equivalent), but words (and numbers) fail us:

Love, to take a leading example, is a feeling that just is. The why and wherefore of it is beyond our ability to understand and explain. Some of the feelings attached to it can be expressed in prose, poetry, and song, but those are superficial expressions that don’t capture the depth of love and why it exists.

The world of science is of no real help. Even if feelings of love could be expressed in scientific terms — the action of hormone A on brain region X — that would be worse than useless. It would reduce love to chemistry, when we know that there’s more to it than that. Why, for example, is hormone A activated by the presence or thought of person M but not person N, even when they’re identical twins?

The world of science is of no real help about “getting to the bottom of things.” Science is an infinite regress. S is explained in terms of T, which is explained in terms of U, which is explained in terms of V, and on and on. For example, there was the “indivisible” atom, which turned out to consist of electrons, protons, and neutrons. But electrons have turned out to be more complicated than originally believed, and protons and neutrons have been found to be made of smaller particles with distinctive characteristics. So it’s reasonable to ask if all of the particles now considered elementary are really indivisible. Perhaps there other more-elementary particles yet to be hypothesized and discovered. And even if all of the truly elementary particles are discovered, scientists will still be unable to explain what those particles really “are.”

Reason is valuable when it consists of the narrow application of logic to hard facts. But it has almost nothing to do with most of life — and especially not with politics.

Just as words fail us, so has the Enlightenment and much of what came in its wake.

As exemplified by this “child of the enlightenment”:

(See also “In Praise of Prejudice” and “We, the Children of the Enlightenment“.)