Kantian ethics refers to a deontological ethical theory developed by German philosopher Immanuel Kant that is based on the notion that: "It is impossible to think of anything at all in the world, or indeed even beyond it, that could be considered good without limitation except a good will." The theory was developed as a result of Enlightenment rationalism, stating that an action can only be right if its maxim—the principle behind it—is duty to the moral law, and arises from a sense of duty in the actor.
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Central to Kant's construction of the moral law is the categorical imperative, which acts on all people, regardless of their interests or desires. Kant formulated the categorical imperative in various ways. His principle of universalizability requires that, for an action to be permissible, it must be possible to apply it to all people without a contradiction occurring. Kant's formulation of humanity, the second section of the categorical imperative, states that as an end in itself, humans are required never to treat others merely as a means to an end, but always as ends in themselves. The formulation of autonomy concludes that rational agents are bound to the moral law by their own will, while Kant's concept of the Kingdom of Ends requires that people act as if the principles of their actions establish a law for a hypothetical kingdom. Kant also distinguished between perfect and imperfect duties. Kant used the example of lying as an application of his ethics: because there is a perfect duty to tell the truth, we must never lie, even if it seems that lying would bring about better consequences than telling the truth. Likewise, a perfect duty (e.g. the duty not to lie) always holds true; an imperfect duty (e.g., the duty to give to charity) can be made flexible and applied in particular time and place.
Those influenced by Kantian ethics include social philosopher Jürgen Habermas, political philosopher John Rawls, and psychoanalyst Jacques Lacan. German philosopher G. W. F. Hegel criticised Kant for not providing specific enough detail in his moral theory to affect decision-making and for denying human nature. The Catholic Church has criticized Kant's ethics as contradictory, and regards Christian ethics as more compatible with virtue ethics. German philosopher Arthur Schopenhauer, arguing that ethics should attempt to describe how people behave, criticised Kant for being prescriptive. Marcia Baron has defended the theory by arguing that duty does not diminish other motivations.
The claim that all humans are due dignity and respect as autonomous agents necessitates that medical professionals should be happy for their treatments to be performed on anyone, and that patients must never be treated merely as useful for society. Kant's approach to sexual ethics emerged from his view that humans should never be used merely as a means to an end, leading him to regard sexual activity as degrading, and to condemn certain specific sexual practices—for example, extramarital sex. Accordingly, some feminist philosophers have used Kantian ethics to condemn practices such as prostitution and pornography, which treat women as means alone. Kant also believed that, because animals do not possess rationality, we cannot have duties to them except indirect duties not to develop immoral dispositions through cruelty towards them.