 We’ll call this first condition the extensionality condition. In the case of a predicate like ‘green’, for example, it requires that the fact which constitutes its meaning determines the appropriate class of things to which ‘green’ is correctly applicable. This will be the class of green things as opposed, say, to the class of grue things (where an object is grue if and only if it is green before some specified time t and blue thereafter), and in the arithmetical case on which we have focussed so far, the extension of ‘+’ will contain the triple \(\langle 57, 68, 125\rangle\) (and not the triple \(\langle 57, 68, 5\rangle\)).