Progress has not followed a straight ascending line, but a spiral with rhythms of progress and retrogression, of evolution and dissolution.
With the growth of knowledge our ideas must from time to time be organized afresh. The change takes place usually in accordance with new maxims as they arise, but it always remains provisional.
A mind, once formed, is never suited after, One yet in growth will ever grateful be. [Ger., Wer fertig ist, dem ist nichts recht zu machen, Ein Werdender wird immer dankbar sein.]
He who moves not forward, goes backward.
The close and thoughtful observer more and more learns to recognize his limitations. He realizes that with the steady growth of knowledge more and more new problems keep on emerging.