The Idea of explaining natural phenomena by appealing to laws of nature is one that is thoroughly familiar to the modern mind. This idea does not perhaps appear quite as natural as it did a century ago, when Engels proclaimed to the mourners at Marx's funeral that just as Darwin had discovered the law of development of organic nature so Marx had discovered the law of development of human history. Twentieth-century historians do not in general conceive their task as including the formulation of laws of history, and the discoveries of modern physics since Maxwell have for the most part been expressed in terms of principles and equations rather than laws. Nevertheless, despite these changes, we are still quite accustomed to thinking in terms of laws of nature; and just because it seems natural it is easy to assume that it is natural for human beings seeking to explain the phenomena of nature to do so by enquiring after the laws by which these phenomena are governed.