 This model of growing levels of productive power, and their associated forms of society, is said to generate a four-stage periodisation of progressive epochs in human history (G. Cohen 1978: 197–201). In the first stage, the material position involves an absence of surplus, and the corresponding social form is a primitive classless society (productive power being too low to enable a class of non-producers to live off the labour of producers). In the second stage, the material position is a surplus large enough to support an exploiting class, but too small to sustain capitalist accumulation, and the corresponding social form is a pre-capitalist class society. In the third stage, the material surplus is moderately high—high enough to support capitalist accumulation—and the corresponding social form is capitalist class society. Capitalism subsequently generates a growing surplus such that it becomes itself unsustainable (for contested reasons), leading to a fourth and final stage, where, on the basis of a precondition of a massive surplus, a modern classless society emerges.