It is well known that Friedrich Engels considered the "relationship between thought and being [Sein], mind and nature' to be the' supreme question in the whole of philosophy'.¹ This was undoubtedly a grave error on his part and one that has affected the subsequent consideration of the history of philosophy. Engels classified all philosophical systems which postulate the supremacy of the mind over nature, as idealistic, and all opposite systems, which postulate the supremacy of nature over the mind, as materialistic.