cease and many never care to proceed further; but at length, if not the individual, at least the generation, catches sight of new heights that must be scaled. The theories that had explained our early difficulties are seen to leave residual difficulties behind. If these cannot also be made to disappear the theory is convicted of incompleteness if not of error. So further work has to be done usually what has been already done has carried us some stages on our way; but it may even be that in order to reach what we now perceive to be the real summit, we have to retrace our steps, come down from our boasted eminence, and humbly at the bottom begin the ascent anew.
Thus the most successful theories have had to submit to reconsideration and revision. The Newtonian theory of gravitation, for example, triumphantly established its claims by showing that a number of unconnected laws which had been suggested by observation were all simple consequences of one great principle. Yet the further question had to be faced: Will that principle explain not only all the general features of planetary motion but also all the minor inequalities? And some of these were at times so stubborn that it was seriously investigated whether the law of gravitation would not have to be modified, or at least some hypothesis added as to the