  The idea is familiar. If what we are interested in is determining the trajectory and velocity of the planets around the Sun, then it is useful to ignore or iron-out a wide variety of features of the system, e.g., the size of the planets, the presence of friction, the gravitational pull that the planets and their moons exert on one another. Here, then, we evidently have a clear-cut case in which “pretending” that certain things are true of the solar system can greatly aid our aim of prediction. As Vaihinger notes, we get the “right” results here because the properties of the system we are ignoring have only a negligible influence on the phenomena we are interested in studying. The solar system behaves nearly just as it would if the planets were mass points, etc. This suggests a counterfactual understanding of fictions: by reasoning from assumptions that are false—sometimes radically false—at the actual world, we may still arrive at conclusions that are true, or quite nearly true, at the actual world. If those results are arrived at more efficiently by entertaining the fiction, then the fiction is justified.