According to Western religion, galaxies are designed by the Creator. Dr. Jin He found many evidences that galaxies are rational distribution of stars. Rational structure in two dimension means that not only there exists an orthogonal net of curves in the plane but also, for each curve, the stellar density on one side of the curve is in constant ratio to the density on the other side of the curve. Such a curve is called a proportion curve or a Darwin curve. Such a distribution of matter is called a rational structure. There are plenty of evidences for rational galaxy structure. We list a few examples. Firstly, galaxy stellar distribution can be fitted to rational structure. Secondly, spiral arms can be fitted to Darwin curves. Thirdly, rational structure dictates New Universal Gravity which explains constant rotation curves elegantly. However, there has been no systematic study on rational structure. This letter presents a general partial differential equation whose solution must be rational structure. Also given in the letter is the geometric meaning of the equation. The general solution to the equation is called the Creator's open quest for humans which has not been answered yet.