The real breakthrough in human flight didn’t come from crafting better wings or using more feathers, even though those are good things. It was brought about by Dutch-Swiss mathematician Daniel Bernoulli and his book Hydrodynamica, a study of fluid mechanics. In 1738, he outlined what was to become known as Bernoulli’s principle, a theory that, when applied to flight, explained the concept of lift. We had gone from correlation (wings and feathers) to causality (lift). Modern flight can be traced directly back to the development and adoption of this theory.