"Neither in science itself, nor in that lower class of the arts which arise out of its practical application, has any individual work an enduring ultimate value, unless from its execution; and this would be altogether independent of its scientific value, and would belong to it solely as a work of art. In science its main worth is temporary, as a stepping-stone to something beyond. Even the Principia, as Newton, with characteristic modesty entitled his great work, is truly but the beginning of a natural philosophy, and no more an ultimate work than Watt's steam-engine or Arkwright's spinning-machine. It may have a lasting interest from its execution, or from accidental circumstances, over and above its scientific value: but, as a scientific treatise it was sure to be superseded; just as the mechanical inventions of one generation, whatever ingenuity they may betoken at the time, are superseded and thrown into the background by those of another. Thus in science there is a continual progress, a pushing onward: no ground is lost; and the lines keep on advancing. We know all that our ancestors knew, and more: the gain is clear, palpable, indisputable. The discoveries made by former ages have become a permanent portion of human knowledge, and serve as a stable groundwork to build fresh discoveries atop of them: as these in their turn will build up another story, and this again another."