Before we proceed, however, we might consider how Kant proposes to ground the CI's second formulation. Moral imperatives are categorical imperatives. The possibility of a categorical imperative depends on there being some end which is essential to practical reason. The only possible such end is rational nature itself. Rational nature would have to be an end in itself in that it would have to be its own end.
Rational nature exists as an end in itself.
There can be categorical imperatives only if rational nature is an end in itself. There can be moral imperatives only if rational nature is an end in itself. Therefore, there is a moral law only if it has as part of its content that persons be required to respect rational nature as such. Each rational agent necessarily regards his own rational nature as an end in itself. But a person's capacity for rational agency is vulnerable to whether others treat it with respect.