To explain what I mean, let me compare the advance of knowledge among mankind to the course taken by a planet. The false paths on which humanity usually enters after every important advance are like the epicycles in the Ptolemaic system, and after passing through one of them, the world is just where it was before it entered it. But the great minds, who really bring the race further on its course do not accompany it on the epicycles it makes from time to time. This explains why posthumous fame is often bought at the expense of contemporary praise, and _vice versa_. An instance of such an epicycle is the philosophy started by Fichte and Schelling, and crowned by Hegel's caricature of it. This epicycle was a deviation from the limit to which philosophy had been ultimately brought by Kant; and at that point I took it up again afterwards, to carry it further. In the intervening period the sham philosophers I have mentioned and some others went through their epicycle, which had just come to an end; so that those who went with them on their course are conscious of the fact that they are exactly at the point from which they started. This circumstance explains why it is that, every thirty years or so, science, literature, and art, as expressed in the spirit of the time, are declared bankrupt. The errors which appear from time to time amount to such a height in that period that the mere weight of their absurdity makes the fabric fall; whilst the opposition to them has been gathering force at the same time. So an upset takes place, often followed by an error in the opposite direction. To exhibit these movements in their periodical return would be the true practical aim of the history of literature: little attention, however, is paid to it. And besides, the comparatively short duration of these periods makes it difficult to collect the data of epochs long gone by, so that it is most convenient to observe how the matter stands in one's own generation. An instance of this tendency, drawn from physical science, is supplied in the Neptunian geology of Werter.