# Decomposition of a natural light beam into two linearly polarized and mutually perpendicular light beams

I found in an optics text the following statement, which left me somewhat confused:

In general, a natural light beam of intensity $$I_0$$ can be decomposed into two linearly polarized light beams, both perpendicular to each other, without any phase correlation and equal intensity $$I_0/2$$.

But I'm not sure where it comes from... Is it a general law? How could it be demonstrated or, at least, reasoned?