 To put a little flesh on those abstract bones, consider the following elaboration of the First Primacy Thesis (Wright, Levine, &amp; Sober 1992: 19). A given level of development of the productive forces is only compatible with a certain type (or with limited types) of economic structure. Since forces of production tend to develop over time (see the “Development Thesis”), the forces eventually reach a level where they are no longer compatible with the relations of production under which they had previously developed. When such incompatibilities arise, the relations are said to “fetter” the development of the forces of production (G. Cohen 1988: 109–123). This fettering leads to revolution, in which social actors (classes) with the capacity to bring new relations into effect emerge, and revolutionise the economic structure in ways that restores growth in the productive forces.