In the text, it is said that the polarization is just the response of the input electric field so they have [tex]P = \epsilon_0 \chi E[/tex] where P is the polarization and E is the input. This makes sense to me. However, why we need [tex]\epsilon_0[/tex] sitting there? Since [tex]\epsilon_0[/tex] has no dimension, so even there is no [tex]\epsilon_0[/tex], the relation is still the relation b/w input and response. So what's the different if we have [tex]\epsilon_0[/tex] there? And by the way, the book said above equation is given when considering linear case. So what is the relation b/w polarization and field when there exists nonlinearity? Thanks.