 Bolzano recognizes three individuating features of ideas with objects : extension, content, and structure. Loosely speaking, the extension of an idea is “the collection of all the objects standing under it” (Bolzano 2004a: 46); more strictly, it is “that particular attribute of an idea by virtue of which it represents only those and no other objects.” (TS, §66, I:298). While ideas with different extensions, for example, [prime number] and [odd number], are perforce different on Bolzano’s account, this is only a sufficient condition, witness the coextensive but clearly different ideas [even prime number] and [positive square root of 4].