I'll do that in three words: absence of thought.
Now in more than three words ...
As I wrote recently, in my post The Sacred Heart Sits Atop the Sacred Backbone,
The great hallmark of the modern world is just this: no boundaries - fuzziness, blur, formlessness: no borders in life, no definition or backbone in the Church, no belief in the reality of form, and hence no ability to think.
This, I believe, is the distinguishing characteristic of the modern world, though there are many others, which include (according to Hilaire Belloc) atheism, cruelty, slavery to the state and to private corporations, etc. But Belloc, too, sees the Irrational as perhaps the most infernal characteristic of the modern mindset.
Chesterton elaborates ...
As a politician, [the modern man] will cry out that war is a waste of life, and then, as a philosopher, that all life is waste of time. A Russian pessimist will denounce a policeman for killing a peasant, and then prove by the highest philosophical principles that the peasant ought to have killed himself. A man denounces marriage as a lie, and then denounces aristocratic profligates for treating it as a lie. ... Therefore the modern man in revolt has become practically useless for all purposes of revolt. By rebelling against everything he has lost his right to rebel against anything.
Now, of course the term "modern" means more than just this. It can refer to a trend or style or era in politics, art, music, literature, and so forth.
And as I'm very fond of pointing out, Chesterton, who was anything but a modernist in his philosophy, was very much a modernist in his style as a writer of fiction. He is almost the typical "modern writer", as far as style and structure is concerned, when it comes to, say, The Man who was Thursday. That book could only have been written in the 20th Century. No other literary movement, such as Classicism or Romanticism, could have produced it. It's a very "modern" novel.
And we also know of some of the true blessings of the modern world - scientific advancement, increased comfort, the information revolution, freedom of movement, etc.
But Chesterton's use of "modern" is almost always as an adjective that describes today's man in the confusion of his soul - because the inner life of the modern world is what Chesterton looks at, and that inner life is one that has abandoned faith in reason: not "faith and reason", but having faith IN reason and in reason's ability to comprehend reality at all - indeed the modern world must eventually abandon faith even in reality itself.
The sword that divides is simple and comes early in the history of modern thought - as soon as you doubt the reality of things, you are a modern. Once you become too squeamish to make the leap of faith from "I think therefore I am" to "It is - and thank God It is!", you are a modern man and you have abandoned all hope of being able to reason further. For if there are no "things" (separate from you and separate from one another), there are no distinctions and ultimately even the greatest of things, the most distinct of things - being itself - ceases to be. Doubt being and you will end up denying being, and with it the mind's reason and the heart's hope.
And you will be the Modern Man.