The tendency of the rate of profit to fall (TRPF) is a theory in the crisis theory of political economy, according to which the rate of profit—the ratio of the profit to the amount of invested capital—decreases over time. This hypothesis gained additional prominence from its discussion by Karl Marx in Chapter 13 of Capital, Volume III, but economists as diverse as Adam Smith, John Stuart Mill, David Ricardo and Stanley Jevons referred explicitly to the TRPF as an empirical phenomenon that demanded further theoretical explanation, although they differed on the reasons why the TRPF should necessarily occur. Geoffrey Hodgson stated that the theory of the TRPF "has been regarded, by most Marxists, as the backbone of revolutionary Marxism. According to this view, its refutation or removal would lead to reformism in theory and practice". Stephen Cullenberg stated that the TRPF "remains one of the most important and highly debated issues of all of economics" because it raises "the fundamental question of whether, as capitalism grows, this very process of growth will undermine its conditions of existence and thereby engender periodic or secular crises." Causal explanations Karl Marx In Marx's critique of political economy, the value of a commodity is the amount of labour that is socially necessary to produce that commodity. Marx argued that technological innovation enabled more efficient means of production. In the short run, physical productivity would increase as a result, allowing the early adopting capitalists to produce greater use values (i.e., physical output). However, in the long run, if demand remains the same and the more productive methods are adopted across the entire economy, the amount of labour required (as a ratio to capital, i.e. the organic composition of capital) would decrease. Now, assuming value is tied to the amount of labor necessary, the value of the physical output would decrease relative to the value of production capital invested. In response, the average rate of industrial profit would therefore tend to decline in the longer term. It declined in the long run, Marx argued, paradoxically not because productivity decreased, but instead because it increased, with the aid of a bigger investment in equipment and materials. The central idea that Marx had, was that overall technological progress has a long-term "labor-saving bias", and that the overall long-term effect of saving labor time in producing commodities with the aid of more and more machinery had to be a