In opposing the dominant ideas of his time, Marx was led to a resolute relativization of those ideas. The eternal verities of dominant thought appeared upon inspection to be only the direct or indirect expression of the class interests of their exponents. Marx attempted to explain ideas systematically in terms of their functions and to relate the thought of individuals to their social roles and class positions. We must go astray, he believed, "if . . . we detach the ideas of the ruling class from the ruling class itself and attribute to them an independent existence, if we confine ourselves to saying that in a particular age these or those ideas were dominant, without paying attention to the conditions of production and the producers of these ideas, and if we thus ignore the individuals and the world conditions which are the source of these ideas."

Ideas, Marx maintained, must be traced to the life-conditions and the historical situations of those who uphold them. For example, it is not sufficient to state that the ideas of bourgeois writers are the ideas of the bourgeoisie. Distinctions must be made between those ideas that emerge at the beginning of the bourgeois era and those that come at it height. Utilitarian notions in the writings of Helvetius and d'Holback differed from those that made their appearance with James Mill and Bentham. "The former correspond with the struggling, still undeveloped bourgeoisie, the latter with the dominant, developed bourgeoisie."

It is with revolutionary ideas as it is with conservative ideas. "The existence of revolutionary ideas in a particular age presupposes the existence of a revolutionary class." "The ruling ideas of each age have ever been the ideas of the ruling class. When people speak of ideas that revolutionize society, they do but express the fact that within the old society the elements of a new one have been created, and that the dissolution of the old ideas keeps even pace with the dissolution of the old conditions of existence."

The ideologists and the political representatives of a class need not share in all the material characteristics of that class, but they share and express the overall cast of mind.

One [must not] imagine that the democratic representatives are indeed all shopkeepers or enthusiastic champions of shopkeepers. According to their education and their individual position they may be as far apart as heaven from earth. What makes them representatives of the petty bourgeoisie is the fact that in their minds they do not go beyond the limits which the latter do not get beyond in life, that they are consequently driven, theoretically, to the same problems and solutions to which material interest and social position drive the latter practically.
Moreover, Marx granted that particular individuals might not always think in terms of class interests, that they "are not 'always' influenced in their attitude by the class to which they belong." But categories of people, as distinct from individuals, are so influenced.

In his more polemical writings Marx used his functional analysis of the relations between ideas and the social position of their proponents as a means of unmasking and debunking specific opponents and specific ideas. His aims were wider, however. Karl Mannheim perceived this when he wrote:

[Marx's] undertaking . . . could reach its final goal only when the interest-bound nature of ideas, the dependence of 'thought' on 'existence,' was brought to light, not merely as regards certain selected ideas of the ruling class, but in such a way that the entire 'ideological superstructure' . . . appeared as dependent upon sociological reality. What was to be done was to demonstrate the existentially determined nature of an entire system of Weltanschauung, rather than of this or that individual idea.
In Marx's later writings, and in particular in a remarkable series of Engels' letters that date from the 1890's, some of the sharp edges of earlier polemical writings were smoothed out. Marx and Engels were now led to repudiate the idea that the economic "infrastructure" alone determined the character of the "superstructure" of ideas and only held onto the assertion that it "ultimately" or "in the last analysis" was the determining factor.

According to the materialist conception of history, the ultimately determinant element in history is the production and reproduction of real life. . . . Hence if somebody twists this into saying that the economic element is the only determining one, he transforms that proposition into a meaningless, abstract and senseless phrase. The economic situation is the basis, but the various elements of the superstructure . . . also exercise their influence upon the course of the historical struggle and in many cases preponderate in determining their form.
In their later writings, both Marx and Engels were led to grant a certain degree of intrinsic autonomy to the development of legal, political, religious, literary, and artistic ideas. They now stressed that mathematics and the natural sciences were exempt from the direct influence of the social and economic infrastructure, and they now granted that superstructures were not only mere reflections of infrastructures, but could in turn react upon them. The Marxian thesis interpreted in this way gained considerable flexibility, although it also lost some of its distinctive qualities.

From Coser, 1977:53-55.