This is mainly of historical interest, though I found a few bits fascinating. In chapter 3 Mill tries to answer the question:
…in regard to any supposed moral standard—What is its sanction? what are the motives to obey? or, more specifically, what is the source of its obligation? whence does it derive its binding force?
He says the answer must always come from either “external sanctions” like punishment and social pressure, or “[t]he internal sanction of duty” which is “a feeling in our own mind; a pain, more or less intense, attendant on violation of duty, which in properly cultivated moral natures rises, in the more serious cases, into shrinking from it as an impossibility.” He knows this sense of duty is present in different forms and degrees in different individuals, but interestingly, he thinks there’s a sort of evolutionary pressure on cultures (he doesn’t use those terms) that leads to a utilitarian-esque sense of duty becoming increasingly prevalent over time. The gist is: we naturally see ourselves as members of a society, and
…society between human beings, except in the relation of master and slave, is manifestly impossible on any other footing than that the interests of all are to be consulted. Society between equals can only exist on the understanding that the interests of all are to be regarded equally. And since in all states of civilization, every person, except an absolute monarch, has equals, everyone is obliged to live on these terms with somebody; and in every age some advance is made toward a state in which it will be impossible to live permanently on other terms with anybody.
That seems like a rather simplistic (and biased) view of history. But I do like the idea that there may be something inherently unstable in trying to give equal consideration to just some subset of people, an instability that creates continual pressure to expand the moral circle.
My copy of the book includes a speech Mill gave on capital punishment. I was mildly surprised to find he was defending capital punishment. He argues that:
Thus, he believes the death penalty reduces the criminal’s suffering and reduces crime in society, making it a double win on utilitarian grounds.
There’s a glaring omission here: he doesn’t consider the suffering caused to anyone who loves the criminal. A parent, for example, might find it much more traumatizing to see their child executed than imprisoned indefinitely.
I also think there’s a paternalistic element in Mill’s argument which should give us pause. If we were really interested in reducing cruelty to the criminal, we could let them choose whether to be executed or imprisoned. If Mill is reluctant to offer such a choice, it suggests that his primary motivation for endorsing capital punishment is the deterrent effect. His claim that execution is also better for the criminal themselves seems more like a cheap attempt to bolster his case than a rigorously-thought-out argument.
The main difference I have with Mill on this, though, is just that I think wrongful convictions are dramatically more common than he assumes. In the Britain of his time, he says:
Our rules of evidence are even too favorable to the prisoner; and juries and Judges carry out the maxim, “It is better that ten guilty should escape than that one innocent person should suffer,” not only to the letter, but beyond the letter. Judges are most anxious to point out, and juries to allow for, the barest possibility of the prisoner’s innocence.
I lack the historical knowledge to know if this is an accurate representation of his government, or if his position in society gave him a skewed perception (on general principles, I’d guess the latter), but in my own country today it doesn’t seem like “the barest possibility of … innocence” reliably protects people from conviction and harsh punishment. (Also, he’s implying that trying to keep the wrongful conviction rate below 10% is a bit extreme. But when we’re talking about killing innocent people, a 10% error rate would be appalling!)
He does raise an interesting worry: that judges and juries might use a lower standard of evidence for convicting people when they think the punishment is less severe (life imprisonment instead of execution), thus leading to more wrongful convictions. I would suggest this concern is better addressed by trying to educate and persuade people to see imprisonment as the very serious punishment that it is.