 (That is, for any things\(_1\) and any things\(_2\) (if something is one of them\(_1\) if and only it is one of them\(_2\), then they\(_1\) are \(\phi\) if and only if they\(_2\) are \(\phi)\).) This is a plural analogue of Leibniz’s law of indiscernibility of identicals, and as such needs to be restricted to formulas \(\phi\) that don’t set up intensional contexts.