The great inventions of the past and their effect on history form a subject that everyone talks about, and no one thinks about, or precious few ever think hard about. So I consider this a good subject for economic historians to do some hard thinking about now. It is not enough just to schematize the subject philosophically; we should replace and improve some ideas of history that are very popular. Particularly in need of examination is the idea of materialistic Karl Marx, which has spread to such respectable professors and other well-heeled gentlemen that Marx would shudder at his associates—the idea that invention, technologic change, determines economic life and hence all history.