Johannes Kepler Quote
I was almost driven to madness in considering and calculating this matter. I could not find out why the planet would rather go on an elliptical orbit. Oh, ridiculous me! As the liberation in 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 liberation. With reasoning derived from physical principles, agreeing with experience, there is no figure left for the orbit of the planet but a perfect ellipse.
Ch.58, as quoted in John Freely, Before Galileo: The Birth of Modern Science in Medieval Europe (2012) - Astronomia nova (1609)