  But, according to Lewis's analysis, the modal operators ‘□’ and ‘◇’, semantically, are quantifiers over worlds. Hence, (19) might appear to be exactly the concretist truth condition for the denial of (the right conjunct of) (16), i.e., it might appear that, on Lewis's analysis, Algol is not a pet accidentally but essentially; likewise, more generally, any individual and any intuitively accidental property of that individual. 