I found the epistemology of structure offered in Section 2.3 the most interesting of these first two chapters so my post will discuss some issues that arise therein. I’ll quickly summarize some material then follow-up with some (mostly rambling) comments and questions.
Sider says structure is a posit. In particular, he says that posits are more justified when they’re unifying. Accordingly, he distinguishes two sorts of unification:
Ideology. The set of our undefined words/concepts/notions.
Example. The identification of inertial and gravitational mass. The same notion of mass is countenanced in both laws. Instead of containing two notions of mass, there is just one notion: mass.
Unification of Fundamental Principles. The set of our fundamental laws. [Do the ‘fundamental laws’ cover both metaphysical and physical laws?]
Example. The orbiting of the planets was shown to require no new fundamental laws: elliptical orbits follow from the second law and law of gravitation.
He also adds that:
While both sorts of unification seem to count in favor of a posit, too much of the former sort without any of the latter seems rarely to be pursued. We like to keep our posits few in number, but we also want them to obey a small number of fundamental laws, from which much else can be derived (13).
Following Quine, Sider holds that one should believe the ontology of one’s best scientific theory. But Sider goes further and extends this slogan to ideology: regard the ideology of one’s best theory as joint-carving. Thus:
Search for the set of concepts and theory stated in terms of those concepts <I, TI>.
Go back to the example of mass. It seems Sider wants to say that, all things considered, one ideology can be more fundamental than another. For example, an ideology that contains only the word ‘mass’ compared to an ideology which contains both ‘inertial mass’ and ‘gravitational mass’ (but not ‘mass’) will be the more fundamental ideology. And it will be more fundamental because the former is more unifying than the latter without sacrificing theoretical virtue (17). But, as Sider would surely concede, the physics says there is no difference between ‘inertial mass’ and ‘gravitational mass’; indeed, both are equivalent to one another, hence, equivalent to ‘mass’. And if they are equivalent, how, then, could the ‘mass’ ideology be more fundamental than the ‘inertial mass’ and ‘gravitational mass’ ideology? After all, the ‘inertial mass’ and ‘gravitational mass’ ideology has the same consequences about mass as the more fundamental ‘mass’ ideology. Again, I think Sider would just say that the former (inferior) ideology just doesn’t carve quite perfectly at the joints because it contains too much syntactical structure, given that less—i.e., one term: ‘mass’—will do. But again, the two notions of mass are equivalent with the singular notion: mass. So does one ideology really carve more accurately—more better—than the other? Is that what Sider thinks?
There’s another passage that confused me:
The Quinean thought also rationalizes changes in beliefs about what is fundamental. The special theory of relativity led to (at least) two such changes. First, we came to regard electromagnetism as a single fundamental force, rather than regarding electricity and magnetism as separate fundamental forces. [footnote omitted] And second, we came to regard spacetime as lacking absolute spatial and temporal separation. These changes weren’t ontic: changes in which entities are accepted. Nor were they merely doctrinal: changes in view, but phrased in the old terms. The changes were rather ideological: we revised our fundamental ideology for describing the world (16–17, emphasis mine).
These changes weren’t ontic? I would have thought that they were. For example, the ideological shift from ‘space’ and ‘time’ to ‘spacetime’ seems to yield the following ontic shift: there being two kinds of substances (space and time) to there being only one kind of substance (spacetime). If this isn’t right, I guess it isn’t clear to me what exactly generates the ontic changes. Is it the ideology, the theory, or is it the pair <I, TI>? Maybe an example will clarify things. Take the following:
Suppose I believe the ontology of our best theory of time which, for sake of example, we’ll say is the Special Theory of Relativity (STR). STR is a scientific theory; it makes certain predictions. The predictions it makes entail new additions to reality and new omissions: things we previously thought to exist don’t exist according to STR. Suppose STR’s new additions to reality include past, present, and future entities and say that STR’s omissions include absolute space, absolute time, temporal asymmetry, intrinsic direction, change, and so on. Now, we would of course say that the changes here are ideological, but wouldn’t we also say that they are ontological as well? Our set of concepts is the ideology and STR is our theory; STR is stated in terms of those concepts and those concepts have ontological implications. [Is that right?] Suppose further that we believe STR but we find it lacking in some respects: we don’t think it carves perfectly at the joints because it says nothing about the direction of time (and we think direction is joint-carving). Indeed, we’re just the kind of philosopher who thinks scientific theory isn’t satisfactory until it can reconcile its image with the manifest; and the manifest image suggests that time has a direction. What we do is add to the ideology of STR. Suppose we add a new topological structure that builds-in a direction to time (à la Tim Maudlin’s Theory of Linear Structures). It seems an ontological posit follows from our ideological posit: spacetime now has something it previously lacked: direction.