Capitals of different magnitude, calculated in percentages, or, what amounts
 to the same in this case, capitals of the same magnitude operating for the same
 working-time and with the same degree of exploitation may produce very much
 different amounts of profit, because of surplus-value, for the reason that a
 difference in the organic composition of capital in different spheres of
 production implies a difference in their variable part, thus a difference in
 the quantities of living labour set in motion by them, and therefore also a
 difference in the quantities of surplus-labour appropriated by them. And this
 surplus-labour is the substance of surplus-value, and thus of profit. In
 different spheres of production equal portions of the total capital comprise
 unequal sources of surplus-value, and the sole source of surplus-value is
 living labour. Assuming the same degree of labour exploitation, the mass of
 labour set in motion by a capital of 100, and consequently the mass of
 surplus-labour appropriated by it, depend on the magnitude of its variable
 component. If a capital, consisting in per cent of 90c +
 10v, produced as much surplus-value, or profit, at the
 same degree of exploitation as a capital consisting of 10c + 90v, it would be as plain as day
 that the surplus-value, and thus value in general, must have an entirely
 different source than labour, and that political economy would then be deprived
 of every rational basis. If we are to assume all the time that £1 stands for
 the weekly wage of a labourer working 60 hours, and that the rate of
 surplus-value is 100%, then it is evident that the total value product of one
 labourer in a week, is £2. Ten labourers would then produce no more than £20.
 And since £10 of the £20 replace the wages, the ten labourers cannot produce
 more surplus-value than £10. On the other hand, 90 labourers, whose total
 product is £180, and whose wages amount to £90, would produce a surplus-value
 of £90. The rate of profit in the first case would thus be 10%, and in the
 other 90% . If this were not so, then value and surplus-value would be
 something else than materialised labour. Since capitals in different spheres of
 production viewed in percentages — or as capitals of equal magnitude
 — are divided differently into variable and constant capital, setting in
 motion unequal quantities of living labour and producing different
 surplus-values, and therefore profits, it follows that the rate of profit,
 which consists precisely of the ratio of surplus-value to total capital in per
 cent, must also differ.