I'm reading Einstein's paper on special relativity (On the electrodynamics of moving bodies 1905), it gives a derivation of the lorentz transformation. In the derivation, he compares the stationary and the moving system. There is one important thing that I don't understand. It says
"In the first place it is clear that the equations must be linear on account of the properties of homogeneity which we attribute to space and time."
Why the equations must be linear?