Some Passages on Existentialism
Ortega
If all things have given up a large part of their secret to physical science, why does [human life] alone hold out so stoutly? The explanation must go deep, down to the roots. Perchance it is no less than this: that man is not a thing, that it is false to talk of human nature, that man has no nature. ("History as a System," p. 185)
Sartre
Authentic existentialism ... states that if God does not exist, there is at least one being in whom existence precedes essence, a being who exists before he can be defined by any concept, and that being is man, or, as Heidegger says, human reality. ("The Humanism of Existentialism," in Guignon and Pereboom, p. 270)
If man, as the existentialist conceives him, is indefinable, it is because at first he is nothing. Only afterward will he be something, and he himself will have made what he will be. Thus, there is no human nature, since there is no God to conceive it. (ibid., p. 271)
Kierkegaard
A human being is spirit. But what is spirit? Spirit is the self. But what is the self? The self is a relation that relates itself to itself or is the relation's relating itself to itself in the relation; the self is not the relation but is the relation's relating itself to itself. (The Sickeness Unto Death, p. 13)
References:
Guignon, Charles B., and Derk Pereboom, eds. (2001). Existentialism: The Basic Writings. Indianapolis: Hackett Publishing.
Kierkegaard, Søren. (1980). The Sickness Unto Death. Translated by H. V. H. a. E. H. Hong. Princeton: Princeton University Press.
Ortega y Gasset, Jose. (1961). History as a System. In History as a System and Other Essays Toward a Philosophy of History. New York: W.W. Norton and Co.