Before Mill wrote his System of Logic, the system of logic outlined by Aristotle in his Organon (see chapter 2, Aristotle) had been accepted as authoritative. Aristotle’s logic is a system of rules for creating syllogisms, arguments that start with a general premise and reach a conclusion about a particular instance, such as “All men are mortal. Socrates is a man. Therefore, Socrates is mortal.” Mill, however, was an empiricist and believed that all knowledge comes to us through our senses and that we only come to believe in any general principles by experiencing many particular instances that bear them out. Although other empiricist philosophers, such as Locke, had argued that experience is the only basis of knowledge, no one before Mill had attempted to write a system of rules, comparable to Aristotle’s, for how we arrive at general principles by starting with particulars. Mill established a distinction between deductive logic, in which we extrapolate from general principles, and inductive logic, in which we draw conclusions from specific cases. Mill maintained that inductive logic is the true basis of knowledge.

Although Mill defines many different types of induction, the basic principles of his system are fairly straightforward. The inductive method is based on the idea of causation; the goal of induction is to determine what causes something. Mill considers various kinds of evidence and proof, but the essential method for establishing a cause is elimination. If an event happens in one set of circumstances but does not happen in other circumstances that are the same except for one thing, that one thing must be the cause of the event. Complicated phenomena involving a number of causes may be explained using a more elaborate inductive method in which separate causes are identified through deduction, and then their combination is identified through deduction. For example, to explain what causes heart disease, we would use empirical evidence from experiments (i.e., induction) to establish many specific laws governing how diet, genetics, exercise, age, and other factors affect the heart, following which we would use deduction to arrive at a hypothesis for how these laws might work together. Finally, we would verify these hypotheses through induction (more experimentation and examination of empirical evidence).

Although Mill considered induction to be the basis of logic, today induction is not considered part of logic at all. The methods of evidence and proof that Mill wrote about are now considered part of the scientific method, whereas logic proper is limited to deduction.

Mill sees experience as the exclusive and sole source of knowledge. He rejects the idea of what he calls intuitive knowledge, which could apply to any kind of knowledge that the mind grasps immediately and with certainty rather than verifying through observation over a period of time. Intuitive knowledge would include such things as Plato’s Forms or Descartes’ “I think, therefore I am.” However, if the mind cannot intuitively perceive itself as a self, the question arises, what does the self consists of? Mill imagines the body as a permanent potentiality of sensations and the mind as a series of actual and possible states of being. In other words, neither the brain nor the body can be said to be a “person,” in the sense we normally use that word, meaning a stable, consistent, identifiable self. Mill grapples with the problem of how a series of different states or impressions can be aware of itself. Mill observes that a bond seems to exist between the various parts of a series (such as the different states of mind through which a person goes), which allows us to say that these parts are the feelings of a person, who is the same person throughout. This bond constitutes the ego. However, Mill’s argument here seems to depend on the existence of a faculty of perception very much like intuition—our minds apparently intuit the bond between elements in a series.

Experience for Mill is that which can be checked, tested, and proven by careful observation and analysis. Experience must be used to test the inferences we make from experience. Mill observes that the fundamental laws of mathematics and logic, which the supporters of intuitive knowledge had long pointed to as proof that there are some things we know that require no experience, are in fact no more than generalizations from experience. He argues that the law of contradiction, another supposedly innate idea which holds that nothing can be both true and not true, is purely a summary of the inherent incongruity of belief and nonbelief. He maintains that any accuracy of knowledge is only hypothetical, and thus fictitious. He views the law of causation (the fact that every event has a cause) as very important to his inductive system, as a generalization from the experience of an invariable and unconditional sequence. Further, Mill acknowledges only one kind of inference—that which occurs from particulars to particulars—and he uses inference to interpret the record of particular experiences, since they alone provide evidence on which any kind of conclusion can rest.

Mill considered the problem of what human beings do from two different perspectives. First, he observed that certain motives correspond to certain actions in very consistent, even invariable sequences. This fact means that human actions are predictable and that a scientific study of human behavior is possible—from this insight, made by Mill and some of his contemporaries, the modern social and behavioral sciences arose. In particular, Mill observed that human beings always act to maximize their own pleasure. Since this observation is essentially a behavioral law, it would be useless to expect human beings to do otherwise, or to argue with them that they should do otherwise.

However, Mill also examined human actions from an ethical standpoint. On the surface, this second perspective would seem to conflict with the first. Ethics concerns what human beings ought to do and assumes freedom of choice, while the study of human behavior focuses on what human beings actually do and what makes them do it. Mill was able to combine these two perspectives because he believed that the pursuit of pleasure that actually motivates human beings does not necessarily conflict with acting for the general good of society, the greatest good for the greatest number of people. Different kinds of pleasures exist, and we can learn to eschew the baser in favor of the higher. Moreover, Mill saw the study of human behavior as being at the service of ethics. By scientifically studying the effects of human actions, we may discover those actions that most advance the happiness of all. Mill rejects the idea that we know right from wrong intuitively, arguing instead that we must judge our actions by their consequences.

For Mill, government does not exist merely to promote and produce the maximum amount of pleasure, which its citizens like to have. Rather, government must continually seek to educate its citizens so that they pursue the higher, mental pleasures over the lower ones. In fact, it is the government’s responsibility, as well as an individual responsibility, to undertake moral education so that the result may be a good society. This moral education must be implemented with the recognition that people are not merely hedonistic pleasure seekers but that they are progressive by their very nature and desire higher pleasures. Thus, a good government is one that encourages an active participation by all its citizens. A bad government is one that forces its citizens to be passively obedient to the wishes and whims of a ruling elite, no matter how sensible these wishes and whims may in fact be.

Like many of the philosophers who preceded him in the eighteenth and nineteenth centuries, Mill saw the individual as sacred and as taking precedence over the state, in the sense that the state exists for the sake of individuals rather than the other way around. However, unlike Hobbes and Rousseau, Mill’s interest in the individual was not as the individual might exist in a state of nature, before entering into society. Instead, Mill imagined the value of the individual as he or she would become with the proper education in a well-structured society. He sees the individual as filled with various potentials, and it is only in conjunction with society that an individual may develop these potentials so that he or she may benefit the community that he or she inhabits. Mill advocates the active life so that individuals may use their various gifts and talents to promote happiness for the greatest number. He sees the active life for the individual as morally superior to a passive one.