Existentialism is many things, but fundamentally it is a realisation and a response.
- The realisation is that this universe is irrational
- The response is a decision to live in a way that makes it rational
The realisation comes to some but not others. It may come through tragedy, or in thinking, or perhaps from a spontaneous burst of understanding on a subway, laying down to sleep, or staring at a television set. The realisation is that the world in its raw form is a bad joke; there is not rhyme or reason to it, no glorious order or progress, no enlightenment at the end of the dusty road. We simply are; we exist. That is all. Some (see Quenton Cassidy) insist that the existential realisation is a delusion; the universe does so make sense, they say. They often call upon a liar paradox, which states that even in claiming the universe is irrational, one claims a rational truth about it. There is at least one universal truth, and thus, universal rationality.
Existentialists would be inclined to shrug and write the objection off as the questioner's subjective understanding. The feeling of universe's irrationality is intuitive to the existentialist, but forced to argue logically he might say: if the universe is rational, it must have a cause. However, any cause for the universe would be naturally part of the universe. Therefore the universe is causeless, as is everything in it. This is the fundamental barrier of reason, and the root of why existentialists classify scientists and Christians in the same category. They both force the issue of absurdity out of mind by invoking ideals of guiding, immanent, and absolute principles.
The existentialist response is more complex; it does not make sense, in truth, but little in the the existentialist's cosmology does. The response is that even if the universe lacks things like meaning and direction, one's mind is a subject and creates them for itself. A good medium to describe it is a painting. Really, a painting is just a canvas and smeared oils, but in the mind of a subject (as in grammar, a causal force) it becomes a thing of great beauty and emotional power. An existentialist solves the lack of God by making himself god; whereas God creates the world in religion, man creates the world in existentialism. This is not to say an existentialist is solipsistic; the outside world is irrational and pointless, but not necessarily nonexistent. He merely thinks that, in the context of his own life, the most important reality is the one in his own mind. After all, is not love, one of the most prized spiritual states, something of one's own consciousness?
The greatest threat to existentialism is not scientific advances; it is inconceivable that our current rational paradigm will solve the riddle of ultimate mystery that shrouds existence. The greatest threat, rather, is the niggling doubt in the existentialist's mind that he is actually an object (as in grammar, a thing receiving causal action) and that the freedom of his beliefs is illusory. Even strong existentialists and subjectivists would admit that the external world seems to influence them. We could simply be colourful specks of being in the great tumultuous spectrum of reality; in this case the universe is truly nihilistic and any hope of rational existence -- however local or personal -- is dashed.
As an existentialist myself, I respond to this doubt as I responded to my doubts on God so many years ago: yes, it is possible. It is possible that I am nothing; it is possible that the shell of the world is empty; it is possible that I am living in a ersatz fantasy of meaning. Nevertheless, I refuse to wring my hands forever in the shadow of ultimate mystery. One must believe what one must believe to live authentically, and though I don't resent those that contradict my values, they are my life, and I must live them until I can't live them any longer.