It is widely agreed upon that slavery and forced labour are unethical. However, consider the following situation:
A group of explorers, returning home from a tropical island, brings a boat full of captured natives to their homeland to sell as slaves. However, to cushion the moral repercussions of slavery, they use their brainwashing-machines to alter the structures of their captives' brains so that they then are able derive pleasure from manual labour. They then sell the perfectly willing "slaves" off to potential buyers.
Is this ethical?
In a purely utilitarian sense, it seems so. Pleasure is maximized both for the slaves and for the slave-owners.
In a Kantian sense, it seems so as well, at least until one considers his second and third formulations. If we apply Kant's "universalization rule" from his first formulation, we see that a world in which everybody enjoys manual labour is clearly a very productive one. However, when we take into account Kant's "use a person not simply as a means, but as an end in his/herself", we run into a few problems. It seems indeterminate - after the brainwashing, the "slaves" are perfectly willing, and in fact wish to do manual labour, but it also seems that they have been used.
One could also argue that it is unethical because the lack of consent for the brainwashing "dehumanizes" the captives.
If it cannot be argued to be "ethical", is it more so than if they had simply enslaved the natives without brainwashing them? Or is it less so, since they may have destroyed an essential part of their humanity in the process of brainwashing?