 The idea behind AR2 is that islands and incars (see  §1.3)  would seem to be objects of broadly the same sort, namely, objects that go out of existence simply by virtue of changing their orientation with respect to some other thing (the water level in the one case, the garage in the other), without their constitutive matter undergoing any intrinsic change. The idea behind AR3 is that, if there truly are islands but no incars, then this is something that would seem to stand in need of explanation: there would have to be something in virtue of which it’s the case. To think otherwise would be to take the facts about what exists to be arbitrary in a way that they plausibly are not.