The dual-detector argument against the modal theory

The modal theory holds that facts (properties) are identical iff they are necessarily equivalent (coextensive). One of the most prominent arguments against the modal theory is Elliot Sober’s dual-detector argument. According to this argument, the fact that some particular thing is a triangle is distinct from the necessarily equivalent fact that it is a trilateral, since it is only the former fact that causes an output of a certain machine. I argue that the dual-detector argument fails, in part because whatever initial plausibility it has relies on the failure to take into consideration a needed relativisation to times and the failure to distinguish between two facts collectively causing a fact and their conjunction singly causing it. I also argue that variants of the argument are equally unsuccessful.