Husserl’s system is ontologically, a traditional double hierarchy. There are regions or spheres of being, and perfectly traditional ones, except that (due to Kant’s “Copernican revolution”) the traditional order is reversed: after the new Urregion of pure consciousness come the region of nature, the psychological region, and finally a region (or perhaps many regions) of Geist. Each such region is based upon a single highest genus of concrete objects (“individua”), corresponding to the traditional highest genera of substances: in pure consciousness, for example, Erlebnisse; in nature, “things” (Dinge). But each region also contains a hierarchy of abstract genera – genera of singular abstracta and of what Husserl calls “categorial” or “syntactic” objects (classes and relations). This structure of “logical modifications,” found analogously in each region, is the concern of logic. In addition, however, to the “formal essence” which each object has by virtue of its position in the logical hierarchy, there are also truths of “material” (sachliche) essence, which apply to objects as members of some species or genus – ultimately, some region of being. Thus the special sciences, which are individuated (as in Aristotle) by the regions they study, are each broadly divided into two parts: a science of essence and a science of “matters of fact.” Finally, there are what might be called matters of metaphysical essence: necessary truths about objects which apply in virtue of their dependence on objects in prior regions, and ultimately within the Urregion of pure consciousness.
This ontological structure translates directly into an epistemological one, because all being in the posterior regions rests on positing Erlebnisse in the realm of pure consciousness, and in particular on originary (immediate) rational theoretical positings, i.e. “intuitions.” The various sciences are therefore based on various types of intuition. Sciences of matters of fact, on the one hand, correspond to the kinds of ordinary intuition, analogous to perception. Sciences of essence, on the other hand, and formal logic, correspond to (formal or material) “essential insight” (Wesensschau). Husserl equates formal- and material-essential insight, respectively, as sources of knowledge, to Kant’s analytic and synthetic a priori, whereas ordinary perceptual intuition, the source of knowledge about matters of fact, corresponds to the Kantian synthetic a posteriori. Phenomenology, finally, as the science of essence in the region of pure consciousness, has knowledge of the way beings in one region are dependent on those in another.
In Carnap’s doctoral thesis, Der Raum, he applies the above Husserlian apparatus to the problem of determining our sources of knowledge about space. Is our knowledge of space analytic, synthetic a priori, or empirical? Carnap answers, in effect: it depends on what you mean by “space.” His answer foreshadows much of his future thought, but is also based directly on Husserl’s remark about this question in Ideen I: that, whereas Euclidean manifold is a formal category (logical modification), our knowledge of geometry as it applies to physical objects is a knowledge of material essence within the region of nature. Der Raum is largely an expansion and explication of that one remark. Our knowledge of “formal space,” Carnap says, is analytic, i.e. derives from “formal ontology in Husserl’s sense,” but our knowledge of the “intuitive space” in which sensible objects are necessarily found is synthetic a priori, i.e. material-essential. There is one important innovation: Carnap claims that essential (a priori) knowledge of intuitive space extends only to its topological properties, whereas the full structure of physical space requires also a choice of metric. This latter choice is informed by the actual behavior of objects (e.g. measuring rods), and knowledge of physical space is thus in part a posteriori – as Carnap also says, a knowledge of “matters of fact.” But such considerations never force the choice of one metric or another: our knowledge of physical space also depends on “free positing”. This last point, which has no equivalent in Husserl, is important. Still more telling is that Carnap compares the choice involved here to a choice of language, although at this stage he sees this as a mere analogy. On the whole, however, the treatment of Der Raum is more or less orthodoxly Husserlian.