If a material object exists, it must have location and duration.
Location implies a three dimensional reference frame in the rectangular coordinates x, y, and z.
The condition of orthogonality requires that any one of the four variables can be changed without affecting any other. The philosophy is that solutions to the given equation must describe all configurations of matter and space that can exist in the physical universe. Since the equation must be satisfied identically for all values of x, y, z, and t, each member of the equation is represented by the same constant. Since this is not the nature of matter, the only remaining choice is that of a negative for K 2 . The conclusion is that the units of K are those of angular frequency.