 Bolzano connected his metaphysical views with the physical doctrines of his time (see section 10). Nevertheless his metaphysical doctrines strike us — at least at first glance — as rather backward. This reproach seems to be justified with respect to the Athanasia. In his other writings (in particular in his Theory of Science), however, Bolzano developed quite modern metaphysical ideas that come close to views of today’s analytic ontology (cf. Berg 1992) and mereology (cf. Krickel 1995). For example, according to the analysis of propositions that Bolzano developed in his theory of science, a time qualification is required in the subject idea of every proposition whose subject (i.e., the object of its subject idea) is a real thing (WL I, 113, 202, 364 f., WL II, 15). This condition is required in order to avoid “Hegelian” contradictions in cases where the object the proposition is about (i.e., its subject) undergoes a change. Sometimes Bolzano restricts this requirement to propositions in which a non-permanent property is attributed to a changeable substance (WL II, 239), since this is sufficient for avoiding the aforementioned contradictions. Due to this proposal, the proposition [Cajus at time \(t_1\) has erudition] and the proposition [Cajus at time \(t_2\) has lack of erudition] can both be true as long as \(t_1\) and \(t_2\) are different; for if \(t_1\) is different from \(t_2\), then the ideas [Cajus at time \(t_1\)] and [Cajus at time \(t_2\)] have different objects (WL I, 202, 365). These remarks indicate that Bolzano conceived of changeable substances such as Cajus as four-dimensional objects, extended in space and time. An idea such as [Cajus at time \(t_1\)] can pick out a time slice of the four-dimensional Cajus; such a time slice or momentary object as Cajus at time \(t_1\) is also an object for Bolzano, but ontologically it is secondary. Considerations such as these result in Bolzano’s proposal to use this property for the definition of time: Time is for Bolzano the condition or qualification that is required in order to make a proposition about a (changeable) real object true or false (WL I, 365).