I was talking about this at a party this afternoon (yes, I do go to the most interesting parties, thanks) and while Scheme is acceptable the Common Lisp is not because it's not OK to go without boolean primitives. Types are a good idea, if you have types the simplest is clearly the boolean, so start there.

I believe firmly that there should be a single true value, which we might reasonably name true, and a single false value, false, other values aren't booleans, so it's no more reasonable to ask whether an empty string is false, than to just forget to close the quote marks on a string. What we wrote isn't a correct program.