 Note that if we think of \(\geq_a\) as a “greater than or equal to” sign, it is the “smaller” world that is either more plausible or else of equal plausibility. The reason for ordering things in this way comes from an idea due to Grove (1988): we think of each world as positioned on the surface of exactly one of a series of concentric spheres (of non-equal radii), with a more plausible world located on a sphere of smaller radius and a less plausible world located on a sphere of greater radius. Consider the following illustration: 