Although materialism has more or less taken hold in the various disciplines as the theory de rigeur,
there is a misunderstanding about postmodernism prevalent in circles that identify as "Orthodox Marxist" in which it is believed to be a rejection entirely of the material viewpoint. It is not, but it is necessary to reconcile the old Marxist doctrine of historical materialism with the postmodern understanding of mediated semiosis; that is, the world is composed of material objects, but those objects cannot be experienced except through a shared social reality, which relies on mediated understandings.
When we talk about materialism and substrate, we must be careful to recognize that social mediators, by which I mean the mental constructs that determine how we understand our interaction with the semi-fictive, unmediated, "real" world, are themselves part of a decent material analysis. We must examine the mental structures that our interactions with the world as material for consideration. Attitudes, mores, etc., are not idealism in Marx's terms, but are rather actual moving parts of the social order. Gramsci recognized this, and the rejection of the so-called Orthodox Marxists that the hegemonic thought-systems constitute a separate and discreet material basis for understanding history is a paucity in the thought of the circles that reject postmodernism wholesale, as a project.
While it remains fundamentally true that material conditions shape ideas, it is also true that ideological formations in return shape material conditions by shaping the ways in which societies can permissibly or even conceptually interact with them. Thus, the change in material reality is typified not solely by a base-superstructure pyramid, but rather a reflux feedback, in which ideological formations in the superstructure are projected downwards onto the base. The one is conditioned on the other, with the base forming the necessarily a priori and fundamental bedrock of ideological formation.
Thus, we may dismiss the simple bourgeois ideology of the "freedom of the press" as being not grounded in material reality, being idealistic. However, the manner in which this idealism is deployed in defense of the material reality (that freedom of the press in a bourgeois democracy is nothing more or less than the freedom of the wealthy to make use of the press as an organ of policy), it is a material fact that we must grapple with in our analysis. The way social behavior is shaped by this ideology is itself part of a new material basis.
What is ideology, then, and idealism, if we reduce the components of ideological arguments to pieces of material experience for analysis? Well, idealism is essentially the claim that there are no mentally-independent realities. But if we examine the classical postmodern works of the Opera Aperta and its partner The Limits of Interpretation, both by Umberto Eco, we can see that postmodernism does not necessarily reject the extant physical world. There are limits to permissible interpretations of the senses, and these limits are determined socially, which creates the social reality in which we live.
Thus, when we as Marxist materialists, "reject idealism", we are not rejecting the notion that ideas have a palpable effect on the world. We are rejecting the naive idealism that considers the stated goals or methodology of ideologies as their actual goals, methodologies, and effects. When we say, for example, "Liberalism is the left wing of fascism," we are actually engaging in complex material analysis of the pragmatic, material effects of classical liberal thought.
The question posed by ultra-materialists, who reject even the very possibility of social constructs, is essentially this one: "Can we interact with the world and receive raw, unmediated, experience?" The answer, of course, is that the very notion of a raw and unmediated experience is fatally flawed and self-contradictory.
In order to understand this, we should examine raw experience as a concept. The ultra-materialist point of view, which rejects socially constructed mediators as acceptable vectors of analysis, must rely on the concept of direct, unmediated, sensory experience. However, anyone who has been socialized in any society must necessarily experience sensation through the lens of these constructed mediators; for example, knowledge of socially-constructed connotations of color, even though they may stem from straightforward-looking connections, are mediators.
The perception of darkness or blackness as being related to death and the grave, for example, is not a universal social mediator. Eastern culture associates whiteness and paleness with death. Both can be logically justified on actual material experiences (the dark is unknown, it is scary, etc.; the dead are pale, etc.)
"Raw experience" thus immediately gives rise to assumptions about perception, which rapidly become social mediators. Social mediators constantly interfere with raw sensory experience, altering the perception of what is experienced. Even were a child to be raised without any social input, the act of generating experiences would give rise to social mediators that the child alone understands. That is, culture and experience exist in a perpetual feedback loop, one affecting the other.
What do we mean by reality, then? We mean the socially undeniable reality that exists as a result of a mass of mediated experiences shared in a social group. We run up, then, against the limits of interpretation much as we would in a fictional work; certain inputs can only be permissibly interpreted in certain ways based on the systems of thought (primarily logic) that we have chosen to adopt as socially useful.
However, the inputs still exist. We can destroy social realities by proving them false, or simply by attacking them vigorously enough until they are no longer supported or no longer exist. However, the material realities that gave rise to those social realities are not necessarily directly accessible. In order to get at them, we must understand and deconstruct the social realities that have arisen as an inevitable consequence of the nature of sense-perception.
Social realities, whether they line up with material realities or not, continue to affect people living in the society, and they must be analyzed with the same degree of care as material realities. They are not, as some ultra-materialists believe, flimsy gauzy veils that simply vanish when material realities change. Certainly, there was a notion of "race" before there was imperialism; however, imperialism empowered race-theory and created a virulent strain of bad social reality that has persisted to this day, even when the most useful elements of bad race-theory have been superseded by neocolonialist attitudes. But because we failed to sweep away the shards of race-theory, it persists and continues to affect people in the world.