John Stuart Mill argues that moral theories are divided between two distinct approaches: the intuitive and inductive schools. Although both schools agree on the existence of a single and highest normative principle (being that actions are right if they tend to promote happiness and wrong if they tend to produce the reverse of happiness), they disagree about whether we have knowledge of that principle intuitively, or inductively. Mill criticises categorical imperative, stating that it is essentially the same as utilitarianism, since it involves calculating the good or bad consequences of an action to determine the morality of that action.
Mill defines "happiness" to be both intellectual and sensual pleasure. He argues that we have a sense of dignity that makes us prefer intellectual pleasures to sensual ones. He adds that the principle of utility involves assessing an action's consequences, and not the motives or character traits of the agent. Mill argues that the principle of utility should be seen as a tool for generating secondary moral principles, which promote general happiness. Thus most of our actions will be judged according to these secondary principles. He feels that we should appeal directly to the principle of utility itself only when faced with a moral dilemma between two secondary principles. For example, a moral principle of charity dictates that one should feed a starving neighbour, and the moral principle of self-preservation dictates that one should feed oneself. If one does not have enough food to do both, then one should determine whether general happiness would be better served by feeding my neighbour, or feeding oneself.
Mill discusses our motivations to abide by the utilitarian standard of morality. Man is not commonly motivated to specific acts such as to kill or steal, instead, we are motivated to promote general happiness. Mill argues that there are two classes of motivations for promoting general happiness. First, there are external motivations arising from our hope of pleasing and fear of displeasing God and other humans. More importantly, there is a motivation internal to the agent, which is the feeling of duty. For Mill, an this feeling of duty consists of an amalgamation of different feelings developed over time, such as sympathy, religious feelings, childhood recollections, and self-worth. The binding force of our sense of duty is the experience of pain or remorse when one acts against these feelings by not promoting general happiness. Mill argues that duty is subjective and develops with experience. However, man has an instinctive feeling of unity, which guides the development of duty toward general happiness.
Mill's proof for the principle of utility notes that no fundamental principle is capable of a direct proof. Instead, the only way to prove that general happiness is desirable is to show man's desire for it. His proof is as follows: If X is the only thing desired, then X is the only thing that ought to be desired. Thus if general happiness is the only thing desired, therefore general happiness is the only thing that ought to be desired. Mill recognises the controversiality of this and therefore anticipates criticisms. A critic might argue that besides happiness, there are other things, such as virtue, which we desire. Responding to this, Mill says that everything we desire becomes part of happiness. Thus, happiness becomes a complex phenomenon composed of many parts, such as virtue, love of money, power, and fame.
Critics of utilitarianism argue that unlike the suppositions of the utilitarians, morality is not based on consequences of actions. Instead, it is based on the fundamental concept of justice. Mill sees the concept of justice as a case for utilitarianism. Thus, he uses the concept of justice, explained in terms of utility, to address the main argument against utilitarianism. Mill offers two counter arguments. First, he argues that social utility governs all moral elements in the notion of justice. The two essential elements in the notion of justice are: punishment, and the violation of another's rights. Punishment results from a combination of revenge and collective social sympathy. As a single entity, revenge has no moral component, and collective social sympathy is equal to social utility. Violation of rights is also derived from utility, as rights are claims that one has on society to protect us. Thus, social utility is the only reason society should protect us. Consequently, both elements of justice are based on utility. Mill's second argument is that if justice were foundational, then justice would not be ambiguous. According to Mill, there are disputes in the notion of justice when examining theories of punishment, fair distribution of wealth, and fair taxation. Only by appealing to utility can these disputes be resolved. Mill concludes that justice is a genuine concept, but it must be seen as based on utility.