 Since every proposition has the same copula, two propositions can be different only if they have either different subject ideas or different predicate ideas or both. This results in Bolzano’s identity criterion for propositions: Two propositions \([A_1\) has \(b_1]\) and \([A_2\) has \(b_2]\) are identical iff (i.e., if and only if) \([A_1] = [A_2]\) and \([b_1] = [b_2]\). In order for \([A\) has \(b]\) to be a proposition, it will suffice that the predicate idea \([b]\) be an arbitrary idea, in a way at least “pretending” to be an idea of an attribute (WL II, 16–18). In order for \([A\) has \(b]\) to be true, however, it is necessary that the predicate idea \([b]\) is an idea of an attribute (Beschaffenheit). An attribute can be an “inner attribute”, i.e., a property (Eigenschaft) of an object, or an “outer attribute”, i.e., a relation (Verhältnis) among objects. Examples of properties are wisdom or omnipotence, examples of relations are friendship to so-and-so, fatherhood to so-and-so, being twice as long as such-and-such (WL I, 378–389).