“Man and generally every rational being exists as an end in himself, not merely as a means to be used by this or that will at its discretion.”Immanuel Kant · Întemeierea metafizicii moravurilor · 1785 · Secțiunea a II-a, AA 4:428
A notebook has a price; a person has dignity — one can be bought, the other never traded for anything else.
Kant here grounds the distinction between objects and persons. Objects have conditional worth, given by someone's desire; a rational being is the source of desires themselves, so it cannot be valued from outside. The mechanism: if worth comes from the capacity to set ends, then the bearer of that capacity cannot herself be merchandise. A notebook has a price because someone wants it; a person is not for someone, but is someone.
The card passage shows why a person cannot have a price. The next step is what follows from it: if a person's value does not come from outside, it cannot be traded away from outside either. You cannot pay someone to give up their dignity, because no equivalent exists. An apartment gets sold when a better offer appears. A promise already given does not get sold when a better offer appears, precisely because the one who promised is the one giving value to offers in the first place. From here a new question opens: what exactly remains forbidden toward a person, even with her consent?
Why it matters The sentence underlies modern human rights and gives us a precise word, dignity, for something we feel but struggle to argue.