Modal properties are always hard to define in everyday language. We can scarcely be sure that a thing exists necessarily, because we cannot confirm whether contradictions to this conclusion lie beyond our perceptual capabilities. As a modal realist, David Kellogg Lewis tackles this problem by reducing modal properties to existential quantifications of objects in possible worlds. Instead of treating modal properties as features possessed by objects, “possibly existing” means that there needs to exist at least one possible world predicated as such that the object is located there, while “possible worlds” are universes existent just like ours but spacio-temporarily disconnected from us. This layout grounds reductions of counterfactual claims in the way that “if A had happened, B would have happened” becomes “in all possible worlds in which A happens, those that are most similar to ours are worlds in which B also happens.” While this adds greater relatability to abstract modality, the author takes problem with the Lewisian similarity of worlds. If this similarity is just another intuitive judgment, Lewis’ modal realism does not count as an improvement to the standard vague modal claims. He can deem two individual things as similar by pointing to a feature X that they share, and attach a “qua-X” criterion to simple similarity. But once there are more than two objects involved, the comparison risks being too cumbersome to holistically carry out. Through the comparative examination of two counterfactual cases, this paper suggest that Lewis has to choose between the laws and the appearances of each world in order to make a claim on the similarity between worlds. And by incorporating an additional dimension of time into the picture, the author argues that the intactness of laws would be more important than the appearance of worlds. She concludes on an improved version of Lewis’s final priority ranking for evaluating similarity between worlds, and emphasize the need to: a) acknowledge explicitly that our preference for laws is a sign that adds importance to the starting stage similarity of the worlds’ appearance; and b) the consistency of the arrow of time is our fundamental assurance for all the similarity comparisons.