"All cats have four legs. Therefore, anything that isn't a cat doesn't have four legs." Maybe Greg was just learning about logical fallacies and hadn't gotten

"All cats have four legs. Therefore, anything that isn't a cat doesn't have four legs." Maybe Greg was just learning about logical fallacies and hadn't gotten all the way through? (Although in that case I don't think his philosophy undergrad was very good, since I think you're supposed to learn that during your bachelor's...) http://en.wikipedia.org/wiki/Denying_the_antecedent http://en.wikipedia.org/wiki/Denying_the_antecedent