" One thing is clear; namely, that since the periodic recurrence of crises
is a product of capitalist society, the causes must lie in the nature of
capital. It must be a matter of a disturbance arising from the specific
character of society. The narrow basis provided by the consumption
relations of capitalist production constitutes, from that point of view,
the general condition of crises, since the impossibility of enlarging
this basis is the precondition for the stagnation of the market. If
consumption could be readily expanded, overproduction would not be
possible. But under capitalist conditions expansion of consumption means
a reduction in the rate of profit. For an increase in consumption by the
broad masses of the population depends upon a rise in wages, which would
reduce the rate of surplus value and hence the rate of profit.
Consequently, if the demand for labour, as a result of the accumulation
of capital, increases so greatly that the rate of profit is reduced, to
a point (at the extreme) where an increased quantity of capital would
not produce a larger profit than did the original capital, then
accumulation must come to an end, since its essential purpose - the
increase of profit - would not be achieved. This is the point at which
one necessary precondition of accumulation, the expansion of
consumption, enters into contradiction with another precondition, namely
the realization of profit. The conditions of realization cannot be
reconciled with the expansion of consumption, and since the former are
decisive, the contradiction develops into a crisis. That is why the
narrow basis of consumption is only a general condition of crises, which
cannot be explained simply by 'underconsumption'. Least of all can the
periodic character of crises be explained in this way, since no periodic
phenomenon can be explained by constant conditions.

[pp. 241-242] "

― , Finance Capital: A study in the latest phase of capitalist development