
Possible worlds semantics is a common tool for doing metaphysics. An interesting question is the question of which properties should be ascribed to the accessibility relation between the possible worlds. Now, surely, we need reflexivity: each possible world can access itself. On standard Kripkean semantics, this guarantees that necessarily true propositions in each world are also true in that world, which is a principle without which possible worlds metaphysics becomes senseless. Moreover, we need the accessibility relation to be transitive. After all, if possible world
w2 is possible from the perspective of possible world
w1, that is, if
w2 is accessible from
w1, and if possible world
w3 is possible from the perspective of possible world
w2, that is, if
w3 is accessible from
w2, then, plausibly and reasonably, possible world
w3 is also possible from the perspective of possible world
w1, which means that
w3 is also accessible from
w1. Indeed, transitivity guarantees that necessity is stable or robust: if a proposition
p is necessarily true in some possible world, then
p is also necessarily necessarily true in that world. Now, what about symmetry? Should we also require that the accessibility relation be symmetric, meaning that if possible world
w2 is accessible from
w1, then
w1 is also accessible from
w2? This doesn't seem to be the case. Here is a proposed counterexample. Consider a possible world
w1 and consider the conjunction
p of natural laws in
w1. Suppose that in
w1 the natural laws are necessarily true, so that
p is necessarily true in
w1. Thus,
p is true in each possible world accessible from
w1, which includes
w1 itself due to reflexivity. Now, consider a possible world
w2 in which
p is false and whose conjunction of natural laws, say
q, thus differs from
p, and in which the natural laws are merely contingently true. So, there is at least one possible world accessible from
w2 in which
q is false and whose conjunction of natural laws thus differs from
q. In this example, since
p is false in
w2,
w2 is, given Kripkean semantics, not accessible from
w1. But plausibly,
w1 is still accessible from
w2. For while the natural laws in
w2 are contingently true and have
q as conjunction, it's from the perspective of
w2 reasonably still metaphysically possible that they could have been necessarily true and have
p as conjunction. That is,
w1 is possible from the perspective of
w2. In this specific plausible case symmetry thus does not hold. It follows that we should not accept that the accessibility relation is symmetric. Therefore, the most adequate modal logic for doing possible worlds semantics within metaphysics is
S4 (reflexivity and transitivity) rather than the stronger
S5 (reflexivity, transitivity and symmetry) or the weaker
KT (only reflexivity).
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