 In addition to destructive ones, there is a second type, the constructive thought experiments. Unsurprisingly, there are many ways they could provide positive support for a theory. One of these is to provide a kind of illustration that makes a theory’s claims clear and evident. In such cases thought experiments serve as a kind of heuristic aid. A result may already be well established, but the thought experiment can lead to a very satisfying sense of understanding. In his Principia Mathematica, Newton provides a wonderful example showing how the moon is kept in its orbit in just the same way as an object falls to the earth (see Ducheyne 2006, pp. 435–437). He illustrates this by means of a cannon shooting a cannon ball further and further. In the limit, the earth curves away as fast as the ball falls, with the eventual result being that the cannon ball will return to the spot where it was fired, and, if not impeded, will go around again and again. This is what the moon is doing. We could arrive at the same conclusion through calculation. But Newton’s thought experiment provides that elusive sense of understanding. It’s a wonderful example of the “aha effect” that is typical of many powerful thought experiments.