Existentialism focuses on single existences, just as the original Marx' Marxism does.
(Nor is it entirely wrong; after all, the best short definition of 'Western Marxism' is that it is the Marxism that takes philosophy seriously.) Sartre and Merleau-Ponty were often quite critical of both the USSR and and also some aspects of Western Marxism (Lukács, e.g.) in their various 'Marxist' works. It is an open-ended philosophy that rejects both the 'utopianism' of the classless society (understood as some final state of human history, i.e., a 'final totalization') and at the same time rejects, avant la lettre, the nihilism of postmodernism. Sartre says, "if such a thing as a Truth can exist in anthropology, it must be a truth that has become, and it must make itself a totalization." He understands that "such a totalization is perpetually in process as History and historical Truth." No totalization, no state of affairs, is permanent; this means that both the dogmas of diamat and the marxisant superstitions regarding some utopian future are categorically rejected. Sartre tells us that there is one question he is posing: "Do we have the means to constitute a structural, historical anthropology?" Well, yes, we have Marxism, and (according to Sartre) within Marxism we have a "philosophy of freedom" (i.e., existentialism) as a sort of loyal opposition. After all, Marxism is, according to Sartre, the 'philosophy of our time'. "And I do not mean to speak only of Communists, but of all the others - fellow travelers, Trotskyites, and Trotsky sympathizers..." Sartre, after mentioning the revolt in Hungary, concludes this line of thought by saying that later, "there was news, a great deal of news; but I have not heard it said that even one Marxist changed his opinion." So we end up with ideal types, 'Soviet bureaucracy' and 'direct Democracy', each the negation of the other. Analysis consists solely in getting rid of detail, in forcing the signification of certain events, in denaturing facts or even in inventing a nature for them in order to discover it later underneath them, as their substance, as unchangeable, FETISHized 'synthetic notions.' The open concepts of Marxism have closed in. Its concepts are dictates; its goal is no longer to increase what it knows but to be itself constituted a priori as an absolute knowledge." But isn't any alliance between Marxism and Existentialism but a pipe-dream? For Sartre, Marxism could come to know Man; but today, "it is precisely the conflict between revolutionary action and the Scholastic justification of this action which prevents Communist man -in socialist countries as in bourgeois countries- from achieving any clear self-consciousness." Marxism was once "the most radical attempt to clarify the historical process in its totality." But "for the last twenty years, on the contrary, its shadow has obscured history..." Is Marxism now dying of 'old age'? In fact, given the never-ending unfolding of a material dialectic, one suspects that there should be no permanence in human history. (Although there can, of course, be stability that endures for a surprisingly long while.) After quoting Engels famous letter to Bernstein regarding 'economic determinism' Sartre says that "we do not conceive of economic conditions as the simple, static structure of an unchangeable society; it is the contradictions within them which form the driving force of history." For Sartre, nothing said above should be taken to mean he opposes Marxism. Far from it: "To be more explicit, we support unreservedly that formulation in Capital by which Marx means to define his 'materialism': 'The mode of production of material life generally dominates the development of social, political, and intellectual life.' We cannot conceive of this conditioning in any form except that of a dialectical movement (contradictions, surpassing, totalizations)." But when does the philosophy of freedom rise that Sartre believes will replace Marxism? But we have no means, no intellectual instrument, no concrete experience which allows us to conceive of this freedom or of this philosophy." The process of human history is ceaseless. In the 'Annexe' to his "Critique of Dialectical Reason" Sartre said of the relation between this slim volume before us and the thousand plus pages of his 'Critique' that he feared "that this mountain of notes might seem to have brought forth a mouse..." This note of mine has been merely a review of the first chapter of this 'mouse'. At some point I would like to scale the 'mountain' (i.e., the "Critique of Dialectical Reason") with a much (much!) longer review... But in closing, I again want to point out again that Marxism, for our author, is the philosophy of today; Sartre adds that tomorrow there will be a philosophy of freedom. Thus the dialectical dance of human culture and inhuman nature must be ceaseless and therefore the 'philosophy of freedom' that, according to Sartre, will one day supplant Marxism will also eventually be overthrown. The question before us is whether our philosophical understanding of Nature (understood ontologically, phenomenologically) and our philosophic understanding of Man (understood dialectically and existentially) can ever be brought together. But that is only half the story; historically (that is, existentially and dialectically) he is certain that virtually every situation humanity finds itself in can be improved. Sartre also believes these two great lines of philosophical thought (i.e., the ontological and the dialectical) must somehow come together. In the long footnote that gobbles up the final pages of the first chapter of "Search for a Method" Sartre says that the "only theory of knowledge which can be valid today is one which is founded on that truth of microphysics: the experimenter is part of the experimental system." What does that mean? It means that "the revelation of a situation is effected in and through the praxis which changes it." Of course, according to Sartre, 'official' Marxism (and perhaps even Marx himself!) knows nothing of this: "Yet the theory of knowledge continues to be the weak point in Marxism. In both cases it is a matter of suppressing subjectivity: with Marx, we are placed beyond it; with Lenin, on this side of it." Sartre regards both these understandings as pre-Marxist! The truth is that subjectivity is neither everything nor nothing; it represents a moment in the objective process (that in which externality is internalized), and this moment is perpetually eliminated only to be perpetually reborn." Does Sartre overcome existential subjectivism in his 'Critique'?
Sartre constructs an alternative to the mechanistic dialectic of many of his contemporary Marxists, which at it's core rejects freedom and flattens historical nuance with pre-determined narratives. Sartre's use of the objective environment (principally, the relations of production) as a partially constitutive, but not fully-determining element of existence, is an important bridge between Existentialism and Marxism, reigning in the practical application of some radical freedom at the core of the former. I think this environmental limitation and the dialectic of its "overcoming" is still applicable today as we see certain forms of politics on the Left dissolving agency in societally-determined identity roles. Today, we should not then forget Sartre's "overcoming" as an essential dialectic that not only rejects environmental determinism, but further allows meaningful individualism within the boundaries of hegemonic societal structures.
Often, the most outrageous and unprovable assertions are delivered at the beginning of a text; thus the entire premises of a philosophical argument appear to me flawed and foolish, and I cant see any reason to go on. Many of the premises he accepts from Marxism appear to me to undermine his argument entirely. In general, I guess Im not equipped to appreciate this book on the level that some others will, and its possible that there are better starter texts for reading Sartre, but it strikes me that this book is a fairly minor footnote in history, and not much more.