Human dignity it seems that you are refering to us longing for happiness. Doesnt animals also long for happiness? But there is no "animal dignity"
The question is why we are allowed to use animals but never humans.
Most Catholic philosophers/theologians seem to believe that we have "Human dignity" since we are made for something bigger/greater ie Heaven (eternal happiness with The Existence itself).
I wonder how that could be proved with only philosophy?
Why do you think we have human dignity?
Or if you even think so!?
I umderstand that one evidence for our dignity is the ability we when it comes to know right from wrong. Some seem better at it than others.
I say that the natural law is a very Good evidence. We often think about and ponder upon if there is something greater than all this.
Still I am not sure that this is an argument against "using" Another person like you can do with animals.
Oftentimes, it seems that we as humans can see greater purposes for actions and object than animals can.
We can by practice see the full purpose of eating, sex and so on. This could be seem as evidence but I am looking for how professomional philosophers have argued.