Final answer:
The main difference is that Mill's utilitarianism judges morality based on the amount of happiness produced by actions, while Kant's deontology is based on the duty to follow universal moral laws irrespective of outcomes.
Step-by-step explanation:
The essential differences between John Stuart Mill’s version of utilitarianism and Immanuel Kant’s deontology are rooted in their fundamental approaches to ethics. Mill’s utilitarianism is a form of consequentialism which suggests that the morality of an action is determined by the outcome, specifically aiming for the greatest happiness for the greatest number. In contrast, Kant’s deontology focuses on the intrinsic duty to adhere to moral laws, asserting that the rightness of an action is grounded in following a universal moral law, regardless of the consequences.
Mill's utilitarianism is not only concerned with the quantity of pleasure but also its quality, thus aligning moral theory with the richer history of ethics. Conversely, Kant's deontological approach establishes the categorical imperative as the guiding principle, where the morality of an action is measured by whether it can be universally applied and respects individuals as ends in themselves, not merely as means to an end.