Explanationism is the idea that prediction is, in itself, insufficient to confirm a theory. To adequately confirm a theory, according to an explanationist, is to see how well it describes events and phenomena that have already been observed. Stephen Brush, a staunch explanationist, would say that a correct prediction does not necessarily confirm the truth of a theory; it could be the case that a theory predicts something and yet does not provide the best explanation of it. Take, for example, the difference in the perspectives of Copernicus and Brahe on the solar system. Copernicus's model of the solar system was heliocentric, positing that all of the planets revolve around the sun. Brahe's theory stated that all of the planets revolved around the sun, except the earth, which was immobile, and that the sun actually revolved around the earth. Even if both accurately predicted future movements of the planets, it is easy to see how Copernicus's theory has less of an 'ad hoc' quality – and, of course, provides a superior explanation of the mechanisms of the solar system. It is certainly true that a theory can successfully predict a certain event, yet fail to provide an adequate explanation for why it happened, or perhaps even stumble on the prediction more by accident than by manner of understanding the mechanism behind the event. A predictionist would argue that while a theory can provide a perfect explanation for something happening, a theory cannot be tested for understanding or explaining the underlying mechanism of a phenomenon unless it can also predict some event that confirms that exact mechanism at work. For instance, a physicist might study the formation of solids and posit that all solids will sink if they are placed in a liquid of the same element, because the solid is denser than the liquid. Given this premise, we discover that the physicist's prediction is true, and even once he stages an experiment with ice and water, he will not be proven wrong, but rather will have discovered a unique property of solid water.