It’s quite a machine, this a priori motorcycle. If you stop to think about it long enough you’ll see that it’s the main thing. The sense data confirm it but the sense data aren’t it. The motorcycle that I believe in an a priori way to be outside of myself is like the money I believe I have in the bank. If I were to go down to the bank and ask to see my money they would look at me a little peculiarly. They don’t have ‘my money’ in any little drawer that they can pull open to show me. ‘My money’ is nothing but some east-west and north-south magnetic domains in some iron oxide resting on a roll of tape in a computer storage bin. But I’m satisfied with this because I’ve faith that if I need the things that money enables, the bank will provide the means, through their checking system, of getting it.
Similarly, even though my sense data have never brought up anything that could be called ‘substance’ I’m satisfied that there’s a capability within the sense data of achieving the things that substance is supposed to do, and that the sense data will continue to match the a priori motorcycle of my mind.