The Metaphysics of Morals by Immanuel Kant Chapter 2 Page 72

not positively with humanity as an end in itself, if every one does not also endeavour, as far as in him lies, to forward the ends of others.

For the ends of any subject which is an end in himself ought as far as possible to be my ends also, if that conception is to have its full effect with me.

This principle, that humanity and generally every rational nature is an end in itself (which is the supreme limiting condition of every man's freedom of action), is not borrowed from experience, firstly, because it is universal, applying as it does to all rational beings whatever, and experience is not capable of determining anything about them; secondly, because it does not present humanity as an end to men (subjectively), that is as an object which men do of themselves actually