If it's worth saying, but not worth its own post (even in Discussion), then it goes here.
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1. Please add the 'open_thread' tag.
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I agree that Kant doesn't seem to have ever considered non-euclidean geometry, and thus can't really be said to be making an argument that space is flat. If we could drop an explanation of general relativity, he'd probably come to terms with it. On the other hand, he just assumes that two straight lines can only intersect once, and that this describes space, which seems pretty much what he was accused of.
I don't see this in the quoted passage. He's trying to illustrate the nature of propositions in geometry, and doesn't appear to be arguing that the parallel postulate is universally true. "Take, for example," is not exactly assertive.
Also, have a care: those two paragraphs are not consecutive in the Critique.