Perhaps in no form of knowledge is personal experience so indispensable as in learning to see that all things are unstable and transitory in this world. There is nothing that, in its own place and for the time it lasts, is not a product of necessity, and therefore capable of being fully justified; and it is this fact that makes circumstances of every year, every month, even of every day, seem as though they might maintain their right to last to all eternity. But we know that this can never be the case, and that in a world where all is fleeting, change alone endures. He is a prudent man who is not only undeceived by apparent stability, but is able to forecast the lines upon which movement will take place.[1] [Footnote 1: _Chance_ plays so great a part in all human affairs that when a man tries to ward off a remote danger by present sacrifice, the danger often vanishes under some new and unforeseen development of events; and then the sacrifice, in addition to being a complete loss, brings about such an altered state of things as to be in itself a source of positive danger in the face of this new development. In taking measures of precaution, then, it is well not to look too far ahead, but to reckon with chance; and often to oppose a courageous front to a danger, in the hope that, like many a dark thunder-cloud, it may pass away without breaking.] But people generally think that present circumstances will last, and that matters will go on in the future as they have clone in the past. Their mistakes arises from the fact that they do not understand the cause of the things they see--causes which, unlike the effects they produce, contain in themselves the germ of future change. The effects are all that people know, and they hold fast to them on the supposition that those unknown causes, which were sufficient to bring them about, will also be able to maintain them as they are. This is a very common error; and the fact that it is common is not without its advantage, for it means that people always err in unison; and hence the calamity which results from the error affects all alike, and is therefore easy to bear; whereas, if a philosopher makes a mistake, he is alone in his error, and so at a double disadvantage.[1] [Footnote 1: I may remark, parenthetically, that all this is a confirmation of the principle laid down in _Die Welt als Wille und Vorstellung_ (Bk. I. p. 94: 4th edit.), that error always consists in making _a wrong inference_, that is, in ascribing a given effect to something that did not cause it.]