In science, each new point of view calls forth a revolution in nomenclature.

Ideas often kindle each other, like electrical sparks.

What is Communism? Communism is the doctrine of the conditions of the liberation of the proletariat. What is the proletariat? The proletariat is that class in society which lives entirely from the sale of its labor and does not draw profit from any kind of capital; whose weal and woe, whose life and death, whose sole existence depends on the demand for labor.

I have learned more [from Balzac] than from all the professional historians, economists, and statisticians put together.

One day we shall certainly 'reduce' thought experimentally to molecular and chemical motions in the brain; but does that exhaust the essence of thought?

Just as Darwin discovered the law of evolution in organic nature, so Marx discovered the law of evolution in human history; he discovered the simple fact, hitherto concealed by an overgrowth of idealogy [sic], that mankind must first of all eat and drink, have shelter and clothing, before it can pursue politics, science, religion, art etc.

It is a curious fact that with every great revolutionary movement the question of 'free love' comes into the foreground.