In a way it is, but in the soul there is also a difference. I have not read Sartre in full because I do not take stock in existentialism, which does not have the same meaning as the word used for the moment in politics, which only means "to exist." What I am told, though, by those who have studied him is that Sartre's existentialism consists mainly with the modern (leading to post-modern) idea that all that stuff of soul, of the "other," of God, is at very best an unknowable, and more likely non-existent. In the end, if we are honest about it, we only have ourselves, naked and raw in a random universe. What we are to do about it is have courage - to push the crutch of spiritual belief and / or religion away and stand against the wind, with courage for the sake of this naked truth, which is the only end we can aspire to. That is, truth in the face of naked, un-abstracted reality, with that "other" or companion within us being only a psychological twist that we ultimately use as a tattered, juvenile security blanket.
And thus the meaning of the phrase, "existential angst," because we do not have the courage to face a universe alone. This I know, because I have often felt such loneliness in the middle of the night, when the darkest fears come to wake and there is no one else, no other presence, there to comfort or console. At such times there is no God or spirit, only naked fear. This, the existentialists would say, is the beginning of truth. The hero in such a world remains undaunted, being, in a twist of Greek philosophy, true to his own self, a self that is alone and will parish as all other life on earth parishes.
This I also know because every now and then, such as happened at the beginning of this weekend, such alone-ness affects my waking self as well. This is much worse. In the night, our problems are magnified, as a child magnifies his own small fears and problems into monsters and horrors. In the day, however, we are our adult selves. What we experience then is not the direct fear of the boogey man, or even our immanent destruction by, say, a tornado or hurricane, but rather the pointlessness of it all. With this, the beauty of nature and the good will of others lose their luster and all becomes a flat gray, the epitome of depression. In this we can also see the progression of oppressive government. In the past tyrants used fear to keep people in line - but in the era of existentialism, the totalitarian state was born. Thus we get the public buildings with only functional design; thus we get the cold pragmatism of an "enlightened" society that functions only to function. We may then have less starving people, but no joy,only existence. Thus is the legacy,as I see it, of existentialism and the modern proto-type that excludes primitive notions of spirit and soul.
But, once the darkness is past, we can see the real faults in this line of thinking, which come to look like more and more like thoughts from a deep inner sickness. Going back to the Greeks, we have that mind-bender phrase, "Truth is Beauty, Beauty is Truth..." and now, looking at the existentialist grayness, we begin to see what that means. Untainted by a depressed spirit, depressed either from disease or a certain social milieu,we have beauty. Think of the sparkling wonder that you saw as a child in even the simplest things; now think of the wonders you have seen as an adult and the feelings of awe - of supernatural awe - that they brought to you. We can deny that this sense of beauty came from any spiritual source, but where, then, did it come from? How can such a subtle and non-utilitarian thing be so pervasive and powerful, so powerful that it gives us reason to live?
It is not born of the existential self. There is no room for spirit there, only frank, hard reality which has no illusions. And yet beauty is not an illusion. And this is where spirit lies, and as such, where truth is born, for in beauty we find the essence of things beyond culture. And because we have a frank, practical side that pushes beauty away, we have a second self, a constant companion in beauty and truth. This is our soul, our ground; without it we are not "free to truth," but devoid of truth, set adrift in a world for which we lack basic understanding. It leads to fear and depression. It is the totalitarian state. And it is the opposite of truth - a lie.
Different theologies express this basic notion from different angles, some perhaps better than others. We have pantheism and pantheism in monotheism, and for us Christians, a three-in-one God. I can not wrap up all the meanings in this one phrase of 'beauty in truth and truth in beauty,' but we do get the beginning of the understanding of our separation, our distinctiveness, our partial selves existing, or so it seems, along side other more complete selves within us. Through it all, we are pulled to truth and the One. Reality is not a black dearth of meaning; to the scholar this may seem so, but in beauty we can see that greater meaning is expressed outside and well beyond what we verbally understand as meaning. Because it exists beyond this tack of verbal logic does not mean it does not exist at all - but rather that it is understood in another part of us, in our "soul." In the end, all that existentialist bravery is, is standing guard over ignorance, clinging to a fallacy of perception no matter the cost. FK