 As far as the inner structure of ideas is concerned (WL I, 243 ff.), Bolzano distinguishes simple from complex ideas: A simple idea has no proper parts, whereas a complex idea has. The “sum” (Summe) of proper parts of a complex idea is called its ‘content’ (‘Inhalt’). Due to Bolzano’s peculiar usage of the term ‘sum’ that is restricted — like his usage of ‘collection’ in general — to sets with at least two members, he could not apply his concept of content to all ideas, but only to complex ideas. In order to simplify matters, we will use here the modern concept of a set, allowing for a set to be a singleton (i.e., containing only one single member) or even to be empty (i.e., containing no member at all). In what follows we will therefore take the content of an arbitrary idea to be the set of all of its parts (including improper ones, i.e., including itself). The content of a simple idea \(i\) is then the singleton \(\{i\}\) containing \(i\) itself as its only member. Two complex ideas \(i_1\) and \(i_2\) can have the same content, i.e., the same parts, without themselves being identical, because the common parts of \(i_1\) and \(i_2\) can be arranged in different ways in \(i_1\) and \(i_2\). Bolzano’s favorite example is: [an erudite son of an unerudite father] has the same content, but is not identical with [an unerudite son of an erudite father]; the same holds for \([3^5]\) and \([5^3]\) (WL I, 244). In analyzing an idea, we will in all cases eventually come upon simple ideas (WL I, 263–265). Without explicitly expressing it, Bolzano obviously held the view that every idea is recursively constructed out of simple ideas. Two ideas are therefore identical iff they are constructed out of the same simple ideas in the same way. In order to be able to apply this general idea precisely in concrete cases, we would have to be able to identify the simple ideas and the formation rules involved. Unfortunately, Bolzano informs us about both only by hinting at examples here and there. As examples of simple ideas he mentions [something] (WL I, 447), [has] (WL I, 380, WL II, 18), [non] (WL II, 415), [Wirklichkeit], i.e., [reality] (WL II, 60), and [Sollen], i.e., [ought] (WL II, 69, WL IV, 489).