I have never gone skydiving, and although I don't consider this a weakness, nevertheless sometimes I'm not sure. But then I reassure myself that neither Newton nor Liebniz, the fathers of Calculus, ever had been skydiving. I do my best to disregard the fact that they never really had the opportunity. I try to convince myself that they were smart men - they could have fashioned parachutes and leapt from cliffs. But they didn't, and how could men who invented Calculus overlook such an obvious activity? Clearly they considered and rejected the idea. And that is why I haven't been skydiving.