Marx and Engels met in their early 20ies and remained friends afterward, and cooperated on laying the foundations of what since has been called Marxism. Engels co-wrote The Communist Manifesto with Marx, and also wrote books by himself, notably 'The position of the English working class' and 'Dialectics of Nature'. He also edited and published volumes II and III of Capital after Marx had died.

It is an interesting and ironical fact that without Engels' financial aid Marx could not have written Capital, and that therefore this attack on the foundations of capitalism was funded by a capitalist.