This passage by Friedrich Engels goes to the roots of Marxism:
The materialist conception of history starts from the proposition that the production of the means to support human life… is the basis of all social structure; that in every society… the manner in which wealth is distributed and society divided into classes or orders is dependent upon what is produced, how it is produced, and how the products are exchanged. From this point of view the final causes of all social changes and political revolutions are sought, not in men’s brains… but in changes in the mode of production and exchange – Engels, F. Socialism, Utopian and Scientific” p. 64 in Mendel A. P. (ed.) 1961. Essential Works of Marxism New York: Bantam Books
At least, at first reading it seems to get to the roots; after thinking about it, greater depths begin to appear. Engels says that from this point of view the final causes are to be sought where he locates them, and he is doubtless right. But why does he take this point of view? Even more cogently: Why should we take this one and not another? The passage does nothing to help, telling us only that the materialist conception of history “starts from” the proposition given. Again: Why? Why start from that proposition and not another? Whatever the reason may be, it is one the Materialist Conception of History can never provide since the proposition precedes that theory.
Having taken this point of view, having started from that proposition, the Marxists go on, Engels says, to seek the final causes of all social changes in changes in the mode of production and exchange. If the final cause of these lies here then these changes in the mode of production and exchange can themselves have no cause; if they have a cause then they themselves are not only a proximate cause of social changes, not a final one.
When we seek a guide through the apparent chaos of political and economic life Marxism has much to offer, but it is not all-sufficient. Unable to explain its own existence it cannot account, either, for production. As Engels tells us in his opening sentence, the MCH starts by assuming production; that is to say, taking it for granted.
from Ideological Commentary 37, September 1989.