 The Tractatus’s notion of substance is the modal analogue of Kant’s temporal notion. Whereas for Kant, substance is that which “persists” (in the sense of existing at all times) (Critique, A 182), for Wittgenstein it is that which, figuratively speaking, “persists” through a “space” of possible worlds. (Compare the idea of a road that crosses several U.S. States. Such a road might be said, metaphorically speaking, to “persist” from one State to the next: in such a locution, as in Wittgenstein's, a temporal notion is enlisted to do spatial duty, though in Wittgenstein's case the space in question is logical rather than physical space and persistence amounts to reaching through the whole of logical space.) Tractarian substance is the “unchanging” in the metaphorical sense of that which does not undergo existence change in the passage (also metaphorical) from world to world. Less figuratively, Tractarian substance is that which exists with respect to every possible world. For Kant, to assert that there is substance (in the schematized sense of the category) is to say that that there is some stuff such that every existence change (i.e., origination or annihilation) is necessarily an alteration or reconfiguration of that stuff. For Wittgenstein, analogously, to say that there is substance is to say that there are some things such that all “existence changes” in the metaphorical passage from world to world are reconfigurations of them. What undergo “existence changes” are atomic states of affairs (configurations of objects): a state of affairs exists with respect to one world but fails to exist with respect to another. Those things that remain in existence through these existence changes, and which are reconfigured in the process, are Tractarian objects. It follows that the objects that “constitute the substance of the world” (2.021) are necessary existents. The Tractatus , rather wonderfully, compresses this whole metaphorical comparison into a single remark: “The object is the fixed, the existing [das Bestehende]; the configuration is the changing [das Wechselnde].” (2.0271). “Wechsel,” it should be noted, is the word that Kant expressly reserves for the notion of existence change as opposed to alteration (Critique, A 187/B 230). (Unfortunately, however, whether Wittgenstein had read the Critique in time for this circumstance to have influenced his own phrasing in the Tractatus is unknown.)