Humanity is in the first instance an end in this negative sense: It is something that limits what I may do in pursuit of my other ends, similar to the way that my end of self-preservation limits what I may do in pursuit of other ends. Insofar as it limits my actions, it is a source of perfect duties. Now many of our ends are subjective in that they are not ends that every rational being must have. Humanity is an objective end, because it is an end that every rational being must have. Hence, my own humanity as well as the humanity of others limit what I am morally permitted to do when I pursue my other, non-mandatory, ends.
--Robert Johnson, Adam Cureton, Edward Zalta (ed.)