The thesis of An Introduction to the Principles of Morals and Legislation by Jeremy Bentham is that human behavior analysis should guide penal legislation, because the concept of morality is the philosophical basis of law, and that moral conduct is influenced by the human impulse to seek pleasure and escape pain.

The theme of morality is seen from a psychological point-of-view rather than in the context of Western Philosophy (e.g. Kant, Descartes, Socrates, Christianity, et al.). This is because Bentham wrote An Introduction to the Principles of Morals as a way to find a scientific method in drafting and improving penal legislation.

The other theme, or sub-theme, of the work is the principle of utility, which is the concept of promoting happiness in the greatest number. This principle is where the concept of seeking pleasure and avoiding pain stems from. The principle of utility made morality--i.e. the concepts of what is good and evil--quantifiable.

Actions and motives are influenced by the principle of utility. Therefore, moral and immoral actions could be examined in the context of that principle. If a criminal murders because he enjoys it, though he was influenced by pleasure, it does not lead to the greatest amount of pleasure because of the suffering he causes. If someone killed another to preserve one's life, though that is also an intentional killing, it was done to avoid pain. If one mistakenly shoots another during a hunting trip, though it causes suffering, it was not intentional.