In the first part of Chapter II, Mill responds to the major arguments against utilitarianism. In so doing, he carves out the nuances of his own brand of utilitarianism, such that this chapter may be read both as him defending the existing notion of utilitarianism (particularly the greatest happiness principle) and breaking with its earlier adherents (e.g. Jeremy Bentham).
Mill first devotes a passage to answering what he believes is an objection to utilitarianism formed out of simple ignorance: the argument that utility is in some way a concept divorced from pleasure. Mill sees this as nothing but lack of education on the part of the objectors, because utility is conceptualized exactly as a calculus of pleasure, and nothing else. He supposes that the misconception may have come from people simply hearing the word "utility" and intuiting from its sound that it was something cold and antithetical to matters of pleasure.
This prompts Mill to clearly articulate the central tenet of utilitarian philosophy, which he refers to as the greatest happiness principle. This principle defines the moral content of actions in proportion to their tendency to promote pleasure and mitigate pain, or vice-versa. So, an action that frustrates pleasure or induces pain is morally wrong, and an action that promotes pleasure or mitigates pain is morally right.
At this point, Mill references the older hedonic philosophies associated with Epicurus, and considers the charge that they pervert human nature by reducing it solely to the seeking of pleasure. Mill contends that this critique rests on the faulty premise that human beings are capable of pleasures no greater than the pleasures experienced by animals. When the full scope of human pleasure is encompassed, Mill argues, then the greatest happiness principle appears self-evident.
Yet Mill believes that there is another issue raised by the misguided critique of Epicureanism: the fact that pleasures are only differentiated by degree and duration. This problem is not resolved by simply considering the full range of human pleasures, for human beings would then still be in the same class as lower animals; they would merely be able to feel their pleasures more intensely, or for a longer period of time. This is an issue that Mill admits is merited and is apparent in the philosophies of utilitarianism that came before him - particularly those of Jeremy Bentham.
To resolves this issue, Mill makes a major break with previous utilitarian philosophy by arguing that there are fundamentally different kinds of pleasure. In Mill's account, animal pleasures are lesser than human pleasures of the intellect, thereby creating a qualitative difference that morally divides humans and animals. In so doing, Mill assuages the worry that utilitarianism robs humans of the virtue of their faculties of reason.
There are two main, noteworthy moves made by Mill in drawing his conclusion about the different types of pleasure. First, he submits that the relative value of different pleasures is determined by the consensus of people who have experienced all pleasures considered. He then says that people who have experienced both animalistic pleasures and intellectual pleasures almost unanimously prefer the latter regardless of duration or intensity, and that this gives us reason to believe that they constitute a different kind of pleasure altogether.
Something Mill could be pressed on today is whether or not humans really are a separate moral class than animals. In some sense, the distinction Mill felt compelled to draw is an artifact of his time; no one, much less philosophers, wanted to endorse a moral theory that committed to the statement that humans have the same moral status of animals. Mill solves this problem in his treatise by establishing higher and lower kinds of pleasure.
These days, while certainly no consensus exists, there are more philosophers who endorse the moral status of animals. Mill, while aware that considerations of all sentient beings factor into moral decisions, probably would not grant animals the degree of morality or sentience that science and philosophy would feel comfortable granting them today. In this light, Mill's differentiation of kinds of pleasure seems tenuous.
To Mill's credit, he suggests in the next part of this chapter that this separation of kinds of pleasure is not necessary to his account of morality, because utilitarian calculations would naturally espouse a noble moral character effecting the same ends as the model of higher pleasures would. Of course, Mill probably would not have granted animals the potential for noble character, which would bring us back to the same critique. Animals also interface with our society in a way different from humans, so the dependency of utilitarian theory on societal harmony could pose a problem to animal inclusivity.
This is not to say that utilitarianism is incompatible with an assertion of the moral status of animals. Quite the contrary, the utilitarian principles of acting according to the happiness of all individuals are some of the most compelling grounds for treating animals well. The point being made here is that the finer points of utilitarian theory would have to be modified from Mill's reasoning in order to accommodate animals, before the theory would be useful to this end.
It is worth noting, as an aside, the move Mill makes in stating that the relative value of pleasures is determined by the majority preference of those people who have experienced all pleasures being considered. This is a very democratic conception of the good, in contrast with theories that posit absolute values of different things, irrespective of people's feelings towards them. This abstractly reflects Mill's overarching philosophical emphasis on the role of the individual in determining the course and moral status of society.