Since I am rehearsing bugbears, Richard Chappell has an old post in which he lists three explanations for why our world, as opposed to any other possible world, is actual. His options are: (1) it is brute luck that ours is the actual world, (2) it is by design that ours is the actual world (i.e. some agent with the power to decide which possible world is actual chose that it be ours), and (3) it is necessary that ours is the actual world; either (a) this is a brute modal fact, or (b) modal realism – the view that all possible worlds exist – is true. (Modal realism, Chappell notices, arguably raises the additional “locative puzzle” of why each one of us – i.e. each of our consciousnesses, as opposed to each of our bodies – is in this possible world, as opposed to some other one. I say it “arguably raises” this puzzle because, as Chappell notes, some have thought the question ill-formed and confused.)
It strikes me that anyone who accepts the simplicity principle – that of any two equally powerful theories we are justified in believing the one that posits fewer entities – cannot accept (1) or (3)-(a). That is, if you accept the simplicity principle you are committed to the view either that it is by design that ours is the actual world or that all possible worlds exist. (Whichever of these views you accept, obviously, must be further supplemented by explanations why the designer made a world in which simple theories tend to be true or why we are located in such a world, respectively.) You cannot accept (1) because brute luck gives you no reason to think that simple theories tend to be true in the actual world – whether they are or not is a matter of…luck.
You also cannot accept (3)-(a), though not for the same reason. If it is a brute modal fact that our world must be the actual world then ex hypothesi it is not luck that ours is the actual world because there is no chance that the actual world could be any world other than ours. But there is a related problem, which is that a brute modal fact is a modal fact for which we have no explanation. A brute modal fact, in other words, is not entailed by our best theories of modality* – it is simply…a brute fact. But (is this right?) there are only two ways to ascertain brute facts: (1) indirectly, by entailment from other facts or (2) directly, by intuition. One cannot ascertain brute facts by (1), which leaves (2). And my intuition, at least, is that ours isn’t the only possible world. So, even if the relevant brute modal fact obtains, we have no reason to think so, and so no reason to trust the simplicity principle.
* A brute modal fact may be entailed by certain epistemic theories – such as that the simplicity principle is true – but that’s only because a particular brute modal fact partially constitutes the basis for the simplicity principle. The simplicity principle, therefore, cannot justify belief in the brute modal fact; it is only justification in believing the brute modal fact that can justify belief in the simplicity principle.