What do you mean by the reason for saying things are extended in space, and enduring though time are contingent applies to space and time as well? Like how would space and time have potentialities in need of actualization or depend on things outside of them to exist?

Alexander wrote:

If stuff is extended precisely in space, it seems absurd to deny extension of space itself.

Well, Descartes, E. J. Lowe, C. B. Martin, and John Heil all think space is what's called an “extended simple”.* So, there are people who argue that space is a single substance (rather than a patchwork of them).

But even if we grant that space is an extended simple, it's still a substance that has properties. So it's still composite.*

Alexander wrote:

[...] it cannot be a good objection [to Leibnizian demands for explanations] to say "maybe space or time (or both) is necessary?". .

Right. I think the lesson here is that we have to be careful about explanation-by-stipulation. Giving something (like space) a label, or insisting (in a foot stomping, pound the table kind of way) that it has (say) the property of being necessary, doesn't really explain anything.

One of Thomists' strong points is that they try to explain the difference between the being of contingent beings and the being of necessary beings. In the case of contingent beings, essence and existence are distinct and need to be unified, conjoined, or “held together”; in the case of necessary beings, essence and existence are identical with one another and therefore don't need to be unified (because they're the same). Thomists have a whole battery of arguments to support their explanation.

(I'll let you guys decide whether “theistic personalists” stomp their feet or explain. :-D)

*Martin and Heil propose that we're contingent modes (tropes) of space. That's how they reduce spatial relations.
*One way “space necessitarians” can get around this argument is by saying that the substance “underlying” and bearing space's properties is what's necessary (even if space's properties and connection with those properties makes space itself contingent).

Alexander wrote:

That's fair, in one sense. But I'm not convinced "extended simple" here is using "simple" in a way that avoids the classical arguments. If it's extended, one bit of space is just not the same as another bit. So it clearly isn't simple in the sense that St Augustine, for example, uses the word "simple" of God - that everything we speak of as in God or a part of God is the same as the whole of God. Space probably is simple in the sense that space is not a mixture of various elements - it isn't composite in the sense that an atom or molecule is composite - so perhaps that is closer to what these philosophers mean.

If space were simple in God's sense, it couldn't have properties. So it's not simple in God's sense.

I say this means space isn't a simple in the mereological sense, either. (Space is the substance, s, + the complex property of being space. s and being space are its proper parts.)*

Don't the "space necessitarians" have to flesh out an answer to "what underlies space?" before this becomes an objection to, rather than just a way of avoiding, arguments for God's existence?

They do. It's not at all clear to me that they can without either introducing composition in the form of a necessary existence property, or baldly asserting that space is necessary in a pound-the-table kind of way. (“s is necessary because s is necessary!”)

*I think Lowe just means that space is one substance, not a patchwork of several, by “extended simple”. (I could be wrong. He could think space is an unstructured, extreme-nominalism-style blob.) I don't know enough about his views on mereology or space to tell why that's all he means though.