2-ism (or one account of it, anyway) states that 2 objects can be distinct even if they share all the same parts and occupy the same space. For example, Lump and Angel share all the same parts and occupy the same space, yet they are two distinct objects. Furthermore, 2 objects can also be distinct even if they share the same historical properties, as long as they differ in other (e.g. modal) properties: Lumpl and Goliath are different objects even if they were both created and destroyed at the same time. For the sake of simplicity, I'll concern myself with the latter case (where all physical and historical properties are shared by the two objects), but this can probably be extended to the former case as well.
The grounding worry is as follows:
(1) Physical properties fully ground all modal properties
(2) If (1), then objects that share all the same physical properties share the same modal properties.
(3) So, objects that share all the same physical properties share the same modal properties ((1), (2))
(4) If (3), then if Lumpl and Goliath share all the same physical properties, then Lumpl and Goliath share all the same modal properties.
(5) So, if Lumpl and Goliath share all the same physical properties, then Lumpl and Goliath share all the same modal properties ((3), (4))
(6) Lumpl and Goliath share all the same physical properties.
(7) So, Lumpl and Goliath share all the same modal properties ((5), (6))
(8) If Lumpl and Goliath share all the same modal properties, then the 2-ist explanation fails.
(9) So, the 2-ist explanation fails ((7), (8))
Quick defense of the premises:
(1) seems like a natural way of thinking of modal properties: the way an object is (physically) will determine the kinds of things the object can be/do.
(2) just expresses the thought that if a fully grounds b, then objects that are identical in terms of a will be identical in terms of b.
(4) is just an example of (2) in action.
(6) is just true by hypothesis. The reason I use Lumpl and Goliath instead of Lump and Angel is to prevent accusations that causal-historical properties count as part of the physical properties of an object: in this case, the causal-historical properties of the two objects are identical as well as all the other physical properties.
(8) is justified as follows: the 2-ist argues that Lumpl and Goliath are different objects because they have different modal properties. (8) just claims that if Lumpl and Goliath don't have different modal properties, then they are not different objects.
It seems like the most plausible premise to deny is (1), which is exactly what Justin did. Justin had an argument for his denial of (1), which goes as follows.
Define Fragility1 as: x is fragile iff it can be shattered by being hit.
Define Fragility2 as: x is fragile iff it can be destroyed by being hit.
(10) if physical properties fully ground all modal properties, then physical properties fully ground both Fragility1 and Fragility2.
(11) if physical properties fully ground Fragility2, then in a debate about whether objects can be destroyed by shattering the physical properties could act as evidence for one view over the other.
(12) It is not the case that in a debate about whether objects can be destroyed by shattering the physical properties could act as evidence for one view over the other.
(13) So, it is not the case that physical properties fully ground Fragility2. ((11), (12))
(14) So, it is not the case that physical properties fully ground modal properties ((10), (13))
I won't bother defending the premises, since that is not actually my concern (someone else is free to challenge them). Suppose we take the conclusion of the argument, and accept that (1) has been plausibly denied. We still have the grounding question: if physical properties don't fully ground modal properties, what does? We could just say that modal properties are brute, but Justin has another way out which might be a little better.
Justin proposed that what grounds the modal properties of something might be the type of thing it is. In other words, Lumpl is the type of thing lump, which includes the modal property of being able to survive squishing ('squishability'). Goliath is the type of thing statue which does not include the modal property of being able to survive squishing ('non-squishability'). Problem solved?
Another Grounding Problem:
Suppose that what grounds the modal property of non-squishability that Goliath has is the sortal property statue. What, then, grounds the sort statue? We can't say that it is physical properties, since Lumpl shares all its physical properties with Goliath. So if physical properties grounded the sort statue, then Lumpl would be a statue, and thus have all the same modal properties that Goliath has (which would in turn entail that the 2-ist is wrong by premise (8)). Maybe the way here would be to say that sortal properties like statue are brute. This is not to concede defeat to the original argument: maybe these sortal properties are less mysterious than modal properties. Whatever view we take, we had better not accept:
(15) Physical properties fully ground all sortal properties.
Even though the 2-ist is forced to reject (15), it seems strange to say that sortal properties such as statue are not grounded at all in physical properties. So the end result might look like:
(17) (Physical properties and x) fully ground sortal properties.
Where x is something interesting Justin says in response to this post.