The Relativity of Best Systems
Lewis defends a kind of best system theory both with respect to laws of nature and with respect to mental content: something is a law of nature iff (roughly) it is part of the best theory about our world; somebody believes that snow is white iff (roughly) this is what best makes sense of his behaviour according to our belief-desire psychology.
In both cases, it looks on first sight as if the theory introduces an implausible relativity into its subject matter: We don't want to say that the laws of nature depend on what we happen to find simple (but simplicity is part of what makes a theory good), and we don't want to say that what someone believes and fears depends on what we think about his behaviour.
In both cases, it is hard to spell out this implausible relativity. For instance, it is easy to make these counterfactuals come out false on Lewis's account:
If we had different views about simplicity, the laws of nature would be different.
If our belief-desire psychology were different, other people would have different beliefs and fears.
It's interesting that Lewis himself regards the relativity of laws as a major problem, but apparently not the relativity of intentional content. I guess that's because it's more plausible that our notions of belief and desire get their meaning by their role in belief-desire pychology (i.e. in our belief-desire psychology) than that our notion of a law of nature gets its meaning by its role in our view that laws play a fundamental role in providing for the best overall theory of the world. That is, if a creature with a different belief-desire psychology claimed that Karl does not believe that snow is white even though our theory says he does, it's reasonable to say that that creature means something else by "believes that snow is white" than we do. But it's less clear if a creature with different views on what makes a theory simple must mean something else by "law of nature" than we do.
Why must we think that the simplicity of a theory is up to us? I take it that on Lewis' view, syntactic complexity and the naturalness of the referents of primitive predicates are part of what makes a theory simple. These factors aren't 'up to us' in any sense.
Probably there are other factors to consider as well. However, it doesn't seem *obvious* to me that 'what strikes us as simple' will play a role in the full analysis of simplicity. Any reasons for thinking that it must be so?