I remember someone on slashdot replied to that quote by saying, basically, If you're a mid-19th century member of Parliament, and you're presented with a machine design you don't understand (and don't expect to), by a respectable person asking for money, but you have a basically non-mysterious view of the world, what kinds of questions can you ask to determine whether the idea is scam? The question Babbage complains about is an excellent one.

If the designer claims it can get the right answers no matter what you've input into the machine, thus relying on reading your mind, you can know it's just a crude attempt at scamming you.