rational or irrational, that is the question

According to Kant, everything rational has intrinsic moral worth. Kant describes this idea that every rational being exists as an end-in-oneself and not merely as a means to an end. Therefore, animals are not intrinsically worthwhile because they cannot be rational. Act so that you treat humanity, whether in your own person or that of another, like an animal, always have an end and never a means. Kant stresses that human beings are free creatures, free to choose, have intelligent thoughts and rationalize. However, he claims that animals are not humans by any means and our duties towards then are indirect duties towards the human race. In essence, to spite an animal, you spite a human. He means that we may not “use” animals as means to our selfish ends. They are not objects or instruments to be used. And to truly judge a man we can do that through “his treatment of animals.”

I agree with Kant’s argument that we learn value towards one another through kindness to all creatures, and that “our duties towards animals are merely indirect duties to human nature.” However, my question is whether or not it is wrong to use another human being as a means. He does state that to use a human being is to disrespect humanity, but does not mention anything about using them as a means. An example of my argument would be to own a human being, or be a slave owner. Humans are rational and to use another human being is irrational according to Kant. By using the slave they become an instrument. If I were to ask Kant this, he would most likely respond that as long as each side consents to the transaction then both sides are respecting the humanity of the other person. However the slave would not consent. Therefore, this agreement would go against Kant’s idea because it denies the basis to be free and have moral intrinsic worth. It denies the moral status of the person as an end in themselves. Hopefully this idea makes a little sense.