"The rate of profit is the motive power of capitalist production. Things are produced only so long as they can be produced with a profit."
and
"The stupendous productivity developing under the capitalist mode of production relative to population, and the increase, if not in the same proportion, of capital-values (not just of their material substance), which grow much more rapidly than the population, contradict the basis, which constantly narrows in relation to the expanding wealth, and for which all this immense productiveness works. They also contradict the conditions under which this swelling capital augments its value. Hence the crises."
(http://marxists.org/archive/marx/works/1894-c3/ch15.htm)
Both seem to refer to the FROP as an erosion of the motive for investment, rather than of the capacity to invest.
So the FROP would be completely consistent with the top-heaviness theory of capitalist crisis -- that there is a shortage of profitable investment opportunities in production. Defenders of the FROP theory would not deny that top-heaviness exists. They would say that top-heaviness is only a symptom and you need the FROP theory to know why top-heaviness arises and how it relates to the rest of the economic phenomena.
Isn't this what Marx is trying to work out in all of those difficult passages of Volume III? (I can't tell for sure if he succeeds.)
Tom
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