This reminds me of Kant. Typically the argument is that social interaction is naturally engaged upon by both sides for a purpose -- that is, as means to an end

This reminds me of Kant. Typically the argument is that social interaction is naturally engaged upon by both sides for a purpose -- that is, as means to an end. However, it can be said to be immoral to treat other humans in the same way you would treat, say, a tool. Basically, you can engage in mutually beneficial interactions, but once you begin to interact with people in a way that neglects them (i.e. is only beneficial to you), you cross the line into immoral behavior. For example: a typical job is a mutually beneficial arrangement. One party agrees to work and the other party agrees to pay. In this sense both parties are respecting the others' decision-making capacity. Neither is using the other in a one-sided way. However, in the case of slavery, we see a one-sided relationship in which the slave does not benefit at all, and into which he would never rationally enter. This is a case of treating someone as purely a means to an end.