Claim. Marxian Theory is, in its essence, and in the historical destiny that its essence requires, the theory of human praxis, indeed, the theory of human social self-reproductive practice as a whole, the total self-theory of humanity, the self-science of the human species, superseding the crippling ideologies that have characterized human cognition throughout the entire historical arc of class societies, culminating in the present, self-globalizing, capital-/wage-labor-based class society, the final chapter of the pre-history of humanity.
Marxian theory is the embryonic worldview of democratic-communist humanity, an empirically-grounded, science-based worldview -- which decidedly does not continue what Marx described as the ahistorical, "abstract materialism" that disfigures and holds back the advance of the sciences, and of the productive forces, in the capitalist era -- which does not continue to impose the contra-empirical, atomistic and reductionist metaphysics that characterize the way in which capitalist ideology infects and enfetters the present-day sciences, including "Mathematical Platonist" and related metaphysical fetishisms of contemporary mathematics.
Marx knew all of this, and planned his life's work accordingly, beginning even before his and Engels' first full-length critique of a capitalist era ideology, written in 1847: The German Ideology.
Maximilien Rubel put it this way --
"According to a plan Marx had developed in 1844, political economy was the first of the topics to be investigated, and critical treatment of that theme was to be followed by the "critique of law, morality, politics, etc." The project was to be wound up with a "special work" that would demonstrate the unity of the whole, show the relation between the various parts and finally end with a "critique of the speculative manner in which these subjects have been dealt with until now." In other words, Marx had at that early date envisaged the critique of political economy as being but one brochure among several, all designed as prolegomena to a fundamental work whose thematic construction was to be the systematic critique of bourgeois social institutions and ideologies." [Maximilien Rubel, Rubel on Karl Marx: Five Essays, Joseph O'Malley and Keith Algozin, editors, Cambridge University Press [New York: 1981], p. 190].
Marx's mathematical manuscripts -- an incipient dialectical, immanent critique of the differential calculus -- written much later in Marx's life, numerous comments in the volumes of Capital, and Engels' testimonies in Anti-Duhring and in his Dialectics of Nature manuscript, all attest that modern natural science and mathematics were among the loci of capitalist ideology that they had slated for immanent critique.
The dialectical, immanent critique of ideologies is a task that resides at the heart of Marxian praxis, and is a task to which Marx devoted the greater part of his life-effort, because Marx's materialism is not a crude "Matter-ism", which is really just idealism in disguise, an idealism of the Abstract Idea of Matter.
Marxian theory is a dialectical synthesis of mechanical materialism and dialectical idealism, based upon a withering practical critique of both.
Marxian historical materialism is a psychohistorical materialism, based upon the realization that it is with cognition, and only with cognition, that humanity "practices" -- that humanity can experience, know, shape, and re-shape its reality.
Commodity, Money, Value -- Capital itself -- are not, at root, physical objects, physical materials: they are psychohistorical materials, psychohistorical materialities, formed in the collective, memetic, 'human-phenomic' intersubjectivity of the communal praxis of human societal self-re-production.
In particular, the revolution that we must make out of capitalism -- out of OUR societal practice as the capital-praxis -- is not simply a matter of a neo-Jacobinoid putsch.
Modern humanity must transform its own <<mentalite'>> in the process of the descendant-phase self-disorganizing system that is itself as the declining capital-praxis, to even enable itself to make that revolution at all.
Producing a new scientific and self-scientific world-view, and producing a new world, must go hand-in-hand, or we will never get there.
Just because Marx and Engels were unable to complete the critique of capitalist ideology as a whole that they had planned, does not mean that any Marxian movement, subsequent to their lives, and worthy of the name "Marxian", is somehow "off the hook" if it fails to continue the work that they began.