The three philosophers, Plato, Kant, and Mill, have very contrastive theories of ethics that they follow. First, the ethical theory of Plato, which is a virtue-based eudaemonistic conception of ethics, is a very high aim of both moral thought and action. In this ethical theory, the soul is not attached to the pleasures of the body. With this being said, Plato's primary concern was to develop a way to create the greatest happiness to both the community as a whole and the individual. Plato believed that the key to happiness is to live a just life. This goes hand in hand with his ethical theory dependent on ones virtues and morals. Second, Kant's theory of ethics is one that prohibits certain courses of action. His ethical theory is an example of deontological moral theory. .
The deontological moral theory is "the rightness or wrongness of actions does not depend on their consequences but on whether they fulfill our duty" (Stanford Encyclopedia of Philosophy). With this being said, Kant prohibits such things as murder, theft, and lying, even if it brings more happiness than the alternative. Kant believed that there was an ultimate principle of morality, which was referred to as the categorical imperative. This categorical imperative determines what our moral duties are. Kant has a say in the moral worth of persons as well. He claims that a person's actions determine their moral worth but there is more to it than the action being either right or wrong. Lastly, Mill's ethical theory is that of utilitarianism. Utilitarianism is a "consequentialist theory, meaning it takes the moral worth of the action to be dependent on their consequences" (Parsons). If your action leads to a good consequence, then it is considered a good action. Utilitarianism follows the utilitarian calculus, which is a method of thinking before you act. It is the net pleasure and lack of pain that results from the action or possible action.