“Political economy, in the widest sense, is the science of the laws governing the production and exchange of the material means of subsistence in human society. Production and exchange are two different functions. Production may occur without exchange, but exchange — being necessarily an exchange of products—cannot occur without production. Each of these two social functions is subject to the action of external influences which to a great extent are peculiar to it and for this reason each has, also to a great extent, its own special laws. But on the other hand, they constantly determine and influence each other to such an extent that they might be termed the abscissa and ordinate of the economic curve.

The conditions under which men produce and exchange vary from country to country, and within each country again from generation to generation. Political economy, therefore, cannot be the same for all countries and for all historical epochs. A tremendous distance separates the bow and arrow, the stone knife and the acts of exchange among savages occurring only by way of exception, from the steam-engine of a thousand horse power, the mechanical loom, the railways and the Bank of England. The inhabitants of Tierra del Fuego have not got so far as mass production and world trade, any more than they have experience of bill-jobbing or a Stock Exchange crash. Anyone who attempted to bring the political economy of Tierra del Fuego under the same laws as are operative in present-day England would obviously produce nothing but the most banal commonplaces. Political economy is therefore essentially a historical science. It deals with material which is historical, that is, constantly changing; it must first investigate the special laws of each individual stage in the evolution of production and exchange, and only when it has completed this investigation will it be able to establish the few quite general laws which hold good for production and exchange in general.” 

— Engels, Anti-Dühring, 1877