 swans are blue even though it is possible that some swans are blue. But given (1′), for this to be possible, it would have to be possible that: no swans are blue even though some swans in some Lewis-world are blue.2 And this cannot be, since any blue swan that is parts of a Lewis-world is a blue swan. Was Lewis really committed to denying that there could have contingently been no blue swans? Puzzle 2 depends on the combination of (1′) with (2). As we saw in Puzzle 1, (1′) entails that if it is possible that no swans are blue, it is possible that it is not possible that some swans are blue. Suppose that-as Lewis believed-some Lewis-worlds contain blue swans and others don't. Then by (2), it is possible that no swans are blue, and hence by (1′), it is possible that it is not possible that some swans are blue. But it is possible that some swans are blue, by (1). This combination of claims is inconsistent in the modal logic S5, where what is possibly necessary is simply necessary, and what is possibly impossible is simply impossible. It is thus puzzling to find it seeming to follow so directly from Lewis's commitments. For Lewis generally seems to be friendly towards S5, at least when the modal operators are interpreted as expressing metaphysical necessity and possibility and the sentences we are dealing with are "de dicto" or "purely qualitative", as 'there are blue swans' and 'there are no blue swans' are supposed to be. Puzzle 3 requires us to generalise the pattern of which (1) and (2) are instances. The schema seems straightforward: (3) It is possible that Q Fs are Gs↔df there is a Lewis world such that Q Fs in it are Gs. Here 'Q' is to be replaced with a quantifier like 'some' or 'many' or 'at least two', and 'F' and 'G' with predicates expressing qualitative 2The argument here is of the evidently valid form (φ↔ ψ),^(ξ∧φ) ` ^(ξ∧ψ). properties. Since 'Lewis-world' is itself such a predicate, (4) is an instance of (3): (4) It is possible that at least two things are Lewis-worlds ↔df the