Schulz hasn't been quoted here before, but you might've seen my use of that quote on http://www.gwern.net/Mistakes to which I will add a quote of Wittgenstein making the same quote but much more compressed and concisely:

One can mistrust one's own senses, but not one's own belief. If there were a verb meaning "to believe falsely", it would not have any [meaningful] first person, present indicative.