Is There a Soap Bubble?

From the proposition with regard to anything which I am perceiving that is a soap bubble, there follows the proposition that it is external to my mind. And if this is true of this kind soap bubble, it's certainly true of any other kind, including the kind unicorn. But Hume doesn't want to make that jump or at least more characterization of Kant is that, well, something is causing it, but we can't know anything about that thing. It probably doesn't have anything like the structure.