Book VI argues that the “moral sciences,” meaning the study of ethics and human nature, require the same logical structure as the physical sciences. A discussion of the concepts of liberty, causation, and necessity as they apply to human affairs leads Mill to conclude that human nature is governed by scientific principles that logic can lay bare and that can be used to promote happiness. Mill argues that human psychology and behavior are governed by universal laws, as is the formation of each person’s ethical character. However, these laws cannot be studied directly, through experiment and observation, but can only be known deductively. Mill considers various scientific methods and the extent to which they may be applied to the social sciences. Although many of the methods of the natural sciences do not work for the social sciences, the methods of induction may still be applied to understand causes and effects, such as the effects of a given policy or legislative act. The social sciences can also be approached deductively, by starting with a priori laws of human nature and reasoning based on them. Mill divides the social sciences into two branches: those in which the causes and effects of human behavior are studied in a context that is assumed to be stable, and those sciences that examine progress and historical change.
The most innovative aspect of A System of Logic is its rigorous, systematic explanation of induction. This explanation is a particularly impressive accomplishment given how unscientific and unsystematic induction seems to be at first glance, especially in comparison with the certainty of a deduction or syllogism. For example, in the famous syllogism, “All men are mortal. Socrates is a man. Socrates must be mortal,” if you know for certain that all men are mortal and that Socrates is a man, you can be very certain of your conclusion—that Socrates will die at some point. However, if you work in the reverse direction, which induction requires you to do, you get a much less certain answer. Socrates died, and Socrates was a man, so perhaps all men are mortal—but maybe not. The fact that Socrates’ friends Diabetes and Bursitis also died can be offered as further evidence that all men are mortal, but it still doesn’t prove it. Nevertheless, for those who believe that all our ideas come from experience, induction is the source of every general principle that we think we know, so induction is the foundation on which deduction is based.