 It seems plausible that it is an essential property of gold to have atomic number 79: gold could not have (existed but) failed to have that property. (A substance in another possible world that fails to have atomic number 79 simply isn’t gold, no matter how similar it may otherwise be to the gold of the actual world.) And yet it seems clear that it can only be known empirically that gold has that atomic number. So, while (2) is a necessary truth, what it says cannot be known a priori. For another illustration of this phenomenon, suppose that I point to the wooden desk in my office and say: