One of Althusser's fundamental lessons, and one that remains beyond the controversies about epistemic breaks, the young Marx, and the real Marx, is that Marx's philosophy and politics must be located not at this or that isolated quote or passage, but as traversing the entirety of his work. The condition of immanent causality is a reading of philosophy itself as the immanent unfolding of ideas that are all the more important because they are pervasive, located not in this or that passage, but in the entirety of the work. To some extent Juan Domingo Sánchez Estop's Althusser et Spinoza: Détours et Retours does a similar work on Althusser, searching for Althusser's Spinozism not just in the few well known passages in the ISA essay, Lire Le Capital, and Elements of Self-Criticism where Spinoza is cited by name, but also in the way that Spinoza's thought or practice of philosophy traverses Althusser's work.
There is not one Spinoza behind Althusser's writing, functioning as the secret kernel, as a transitive cause behind the scenes, but different engagements with Spinoza over the course of the trajectory, a Spinoza that exists only in terms of its effects. Sánchez Estop reads Althusser and Spinoza together, reading one by the way of the other. When it comes to Althusser's earliest works on humanism Sánchez Estop draws from Spinoza's attention to the specific history of an enunciation to engaged the specific politics of humanism in the sixties. In other words far from being a common notion, or even a general idea, "humanism" has to be understood as a particular intervention that means different things at different times. The humanism of Feuerbach is not the same as the humanism of the PCF. Just as Spinoza argues that the sense of prophecy must be understood in terms of the understanding and situation of its enunciation, the same is true for the polemics around humanism.
Sánchez Estop cites Althusser in an interview from 1966 stating that "the Tractatus Theologico-Politicus is the Capital of Spinoza, because Spinoza is preoccupied above all with history and politics." This might seem surprising, it would make more sense for Althusser, especially the Althusser of sixties to connect Capital and the Ethics, two systematic works in which their presentation (Darstellung) is integral to their articulation of a "science" that breaks with the imaginary of experience. As Sánchez Estop reminds us when Spinoza first appears by name in Althusser's work it is a "theorist of reading and history," which is another way of saying that for Althusser Capital is first and foremost a critique of political economy through a reading of political economy, a reading that produces a theory of capital through what classical political economy cannot see (and cannot see what it does not see). The reference to Spinoza, rather than Freud, underscores the political dimension of the symptomatic reading. While much of the writing on Althusser and Spinoza focuses on the Ethics, mostly on the Appendix, Sánchez Estop makes the case that the Tractatus Theologico-Politicus is as much of an influence, even if its idea of immanent sense, of making sense of scripture from scripture, is also influenced by the philosophy of science.
Althusser's tendency to draw from multiple sources, psychoanalysis, philosophy, the history of Marxist practice produces its own divisions and tensions in Althusser's text. The tension with the most complex history is that which divides psychoanalysis from the Spinozist elements. At the core of "The Object of Capital" is an attempt to stitch together two concepts, metonymic causality and immanent causality, to think the capitalist structure as simultaneously absent and in its effects. As Althusser writes:
Sánchez Estop argues that these revisions and partial steps are not just aimed at clarifying what is at state in immanent causality, but ultimately drawing together two of Althusser's fundamental concepts of the 60s, immanent causality and overdetermination, structure and conjuncture. As Etienne Balibar describes this trajectory,
"First of all, this conception of an overdetermined causality immediately removes the traditional opposition of "structure" and "conjuncture"; better said, it suggests that these two terms are reciprocal. It is no longer a question of viewing the conjuncture as a short moment in the life of a structure or a transition between successive stages of the structure, because the reality of the structure is nothing but the unpredictable succession of conjunctures; conversely the conjuncture is merely determined as a certain disposition of the structure."
As Sánchez Estop argues the attempt to think conjuncture as structure, or structure as conjuncture, to think an essence as its existence, as its relations, is the basis for a return and renewal of the engagement with Spinoza. As Althusser writes, "In good Spinozist Marxism essence and existence do not exist in two stages: essence only exists in its existence, in the conditions of its existence" ("En bon spinozism-marxisme, l'essence et l'existence n'existent pas à deux étages: l'essence n'existe que dans son existence, dans les conditions de son existence.") In other words to say that a cause exists only in its effects, is to also say that it has no existence outside of its conditions. There is no essence outside or behind the world, but only the actual existence of that which is always already affected or modified. From this perspective Balibar's turn toward transindividuality is not a break with immanent causality, a switch from ontology to anthropology, but a continuation. As I wrote in my preface to Balibar's book,
"Transindividuality is not just an ethical or political value, but ultimately a rethinking of causality, of the way singular things are affected and determined by their relations. There are echoes here of Althusser’s original invocationof Spinoza in Reading Capital as providing a new model of causality, that went beyond linear (or transitive) causality of an empirical type and expressive causality of Hegel. In that text, causality was a matter of thinking the social structure, the mode of production as immanent cause, as a cause which exists only in and through its effects, Balibar’s return to causality is less about an attempt to think the ultimate causality of the mode of production on other structures, but to understand every singular thing as necessarily determined by an intersection of causes andthus the necessary singular nature of every causal relation."
In other words to say the structure only exists in its effects has as its corollary or consequence the assertion that the individual only exists in their relations. This is not to say that immanent cause, conjuncture, and transidividuality are the same, there are necessary shifts and tensions, but they exist as relations of the same problem, or problematic, put to work by Althusser.
This is equally true of Althusser's practice of philosophy, it is a practice that exists in terms of its relations. Spinoza is always thought in relation to Marx, as well as in relation to Machiavelli. It is a Spinoza read through Machiavelli that makes possible a thought of what Althusser referred to as the singular case, in other words to think the singular as nothing other than common relations, and common relations as the expression of the singular. Althusser is most engaging as a thinker when he is producing these conceptual mashups, reading Spinoza with Marx, Mao with Freud, or Machiavelli with Lenin, but that does not mean that they all work. Sánchez Estop goes to great pains to trace Althusser's last encounter with Spinoza, the Spinoza of aleatory materialism. As much as Sánchez Estop does a brilliant job of excavating the understanding of the attributes, the non-relation of ideas and things that underlies Althusser's idea of an encounter, the relation between the aleatory and determination remains more of a non-relation than a relation.
Sânchez Estop's book ends with pictures of Althusser's manuscripts on Spinoza. These notes are partial, a few pages on Spinoza's concept of causality, a few on Machiavelli and Spinoza, and a few on Hobbes and Spinoza. They are perhaps less than one would hope for in terms of quantity and quality, no thorough reading of the Ethics or the TTP, but perhaps that is because Althusser's Spinoza exists in its effects, in the transformations of the concept of ideology, causality, and structure.