New ideas are difficult just because they are new. Repetition has somehow plastered over the gaps and inconsistencies in the old ones, and the new cannot penetrate.

The point of studying economics is so as not to be fooled by economists.

But, as soon as speculators become an important influence in the market, their business is to speculate on each others behaviour.

If there is any law governing the distribution of income between classes, it still remains to be discovered.

science progresses by trial and error, and when it is forbidden to admit error there can be no progress.

Rosa Luxemburg maintained that the capitalist system can keep up its rate of investment (and therefore its profits) only so long as it is expanding geographically.

In all the talk in the Principles (as opposed to the formal analysis) it is not the saving of rentiers but the energy of entrepreneurs which governs accumulation.