“In what are commonly regarded as classics of existentialism–Sartre’s Being and Nothingness, Heidegger’s Being and Time, and Maurice Merleau-Ponty’s Phenomenology of Perception–but also in works by Jaspers, Marcel, Beauvoir, Martin Buber, and José Ortega y Gasset, there is affinity on each of the following issues:
The human predicament that inspires the very enterprise of philosophy; the distinctive character of human existence that distinguishes it from all other types of existence; the intimacy of the relationship between human beings and their world; the radical character of individual human freedom; the tone that a life led in appreciation of this freedom must possess; and the structure of interpersonal relations consonant with this radical, existential freedom.
Affinity with respect to these issues allows, to be sure, for interesting differences among existentialist thinkers. Nevertheless, the affinity is substantial. Each of the writers mentioned in the preceding paragraph would, at some time in their careers, have endorsed the following general statement, a sort of existentialist “manifesto”:
Human beings are prone to experience estrangement from the world in which they live, and it is this sense of estrangement which has long inspired philosophical attempts to locate human existence in relation to the order of things. A sense of estrangement is rooted in the fact that, while human beings are embodied occupants of the world, their powers of reflection, self-interpretation, evaluation, and choice distinguish them from all other occupants of the world–from animals, plants, and mere things. It would be wrong, though, to infer from this distinction that there is no intimate relationship between human beings and the world. Indeed, philosophical reflection on human existence and the world reveals that neither is thinkable in the absence of the other. A main reason for this is that the world of things cannot be understood except by reference to the significance that these things have in relation to human purposes and practices. Once this intimacy is appreciated–and once the sense of estrangement is properly construed–it emerges that each human being is possessed of a radical freedom and responsibility, not only to choose and to act, but to interpret and evaluate the world. Honest recognition by people of the disturbing degree of freedom that they possess requires cultivating a moral comportment or stance towards themselves and others that honours the reciprocal interdependence of individual lives.”
— The Cambridge Companion to Existentialism (Cambridge Companions to Philosophy) by Steven Crowell