The argument would have been disingenuous with respect to the goals of slave masters, who typically seek only their own advantage. But the selfish intent of slave masters is not what makes slavery wrong. Slavery is abhorrent because it robs slaves of their autonomy, forcing them to deploy their labour according to someone else's purposes rather than their own. We need not judge the merits of those purposes to condemn slavery. It remains abhorrent even if the purposes of the slave master (the PM in this case) are superior to those of the slave. 

I do not mean to suggest that Brown's purposes really are superior to those of the teenagers who prefer not to do the work he wants of them. Among these millions of teenagers there must be a great variety of reasons for their failure to volunteer — perhaps they prefer to study or to do paid work or to sit in their bedrooms sniffing glue. I cannot possibly know the reasons each has or whether they are good reasons.

But if I cannot know, how can Brown? What special knowledge does he possess that allows him to judge better than these teenagers and their parents whether they have good reasons to avoid community work? How can he know that, besides violating an anti-slavery principle that should be inviolable except in great emergencies, his proposal will not prevent these teenagers from putting the 50 hours to more valuable use? The PM often recommends himself to us on account of his "moral compass". A man who speaks so confidently of his own virtue can only appal those with a normal sense of modesty. But the vulgarity of Brown's conceit is unimportant compared to its erosion of his restraint.

A morally humble leader is unlikely to force others to live according to his values. Since he is aware that he knows no more about virtue than most of his fellow citizens, and far less about each of their individual circumstances, he will not try to make them act against their own consciences.

But no such modesty inhibits Brown. He believes it is his job to make Britain a more decent and just society and that he is qualified for such an exalted role by the peculiar clarity and strength of his moral convictions. It is a dangerous delusion. Once a politician sees himself thus, he can easily imagine that by compelling people to act as if they shared his values he is bestowing a gift upon them. He can imagine that slavery is permissible provided he is the slave master.