The question puts us in danger of resembling the man who only looks for his dropped watch under the light of a street lamp: the scientific concept that has the widest impact in our lives may not necessarily be the simplest. The Navier-Stokes equations date from 1822 and apply Newton's Second Law of Motion to viscous fluids. The range of applications is vast—in weather prediction, aircraft and car design, pollution and flood control, hydro-electric architecture, in the study of climate change, blood flow, ocean currents, tides, turbulence, shock waves and the representation of water in video games or animations.
The name of Claude-Louis Navier is to be found inscribed on the Eiffel Tower, whereas the Irishman, George Stokes, once president of the Royal Society, is not well known outside of maths and physics. Among many other achievements, Stokes laid the foundations of spectroscopy. It needs a John Milton of mathematics to come down among us and metamorphose the equations into lyrical English (or French) so that we can properly celebrate their ingenuity and enduring use, and revive the reputations of these two giants of nineteenth century science.