Inspired by a comment and a conversation in a mail room.
(1) Suppose there's an apriori argument for either substance dualism (or, more carefully, immaterialism about minds).
(2) We know apriori that such an argument would establish that minds stand in no spatial relations.
(3) We know apriori (well, from the armchair) that minds stand in temporal relations.
(C) We know apriori (or from the armchair) that things can stand in temporal relations without standing in any spatial relations.
That seems to be the sort of thing we cannot know apriori, and not (just) because it seems false.
Can we turn this into an argument against dualism?
(1) If substance dualism were true, minds would stand in no spatial relations.
(2) If minds stood in no spatial relations, it would stand in no temporal relations.
(3) Minds do, however, stand in temporal relations.
(C) Dualism is false.
Solution. Mental events aren't temporally related to anything, not even other mental events. Problem. I just thought of that response, I thought of the argument much earlier.
Can the dualist challenge (1) or do they just put their faith in the idea that there can be temporal relations of things like simultaneity between things that are spatial and things that are not even though we cannot assess temporal relations between spatial things apart from some frame of reference?