  It seems obvious that “water” doesn’t mean the same as “H\(_2\)O” (and all the more so if we take speakers as authoritative as to what their words mean). For it was a discovery when 18th century chemists figured this fact out. But being water and being H\(_2\)O are not just a case of a pair of coextensive properties, such as being a cordate and being a renate. Being water and being H\(_2\)O are one and the same identical property, the property identity in question being a posteriori, not a priori and certainly not analytic. So the Open Question Argument cannot refute Metaphysical Naturalism. At most, the Open Question Argument gives us a reason to be synthetic, rather than analytic, naturalists.