I do not postulate a kind of pre-existent harmony between the different levels of structure. They may be – and often are – completely contradictory, but the modes of contradiction all belong the same type. Indeed, according to dialectic materialism it should always be possible to proceed, by transformation, from economic or social structure to the structure of law, art, or religion. But Marx never claimed that there was only one type of transformation – for example, that ideology was simply a “mirror image” of social relations. In his view, these transformations were dialectic, and in some cases he went to great lengths to discover the crucial transformation which at first sight seemed to defy analysis.
From Structural Anthropology, pp. 329-330.