I was almost driven to madness in considering and calculating the matter. I could not find out why the planet [Mars] would rather go on an elliptical orbit. Oh ridiculous me! As if the libration on the diameter could not also be the way to the ellipse. So this notion brought me up short, that the ellipse exists because of the libration. With reasoning derived from physical principles agreeing with experience, there is no figure left for the orbit of the planet except for a perfect ellipse. …

Why should I mince my words? The truth of Nature, which I had rejected and chased away, returned by stealth through the back door, disguising itself to be accepted. That is to say, I laid [the original equation] aside, and fell back on ellipses, believing that this was a quite different hypothesis, whereas the two, as I shall prove in the next chapter, are one and the same. … I thought and searched, until I went nearly mad, for a reason why the planet preferred an elliptical orbit. … Ah, what a foolish bird I have been!