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Kantian ethics refers to a deontological ethical theory ascribed to the German philosopher Immanuel Kant. The theory, developed as a result of Enlightenment rationalism, is based on the view that the only intrinsically good thing is a good will; an action can only be good if its maxim – the principle behind it – is duty to the moral law. 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. If a contradiction occurs the act violates Aristotle's "Non-contradiction" concept which states that just actions cannot lead to contradictions. 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, additionally, 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. A perfect duty, such as the duty not to lie, always holds true; an imperfect duty, such as the duty to give to charity, can be made flexible and applied in particular time and place.
American philosopher Louis Pojman has cited Pietism, political philosopher Jean-Jacques Rousseau, the modern debate between rationalism and empiricism, and the influence of natural law as influences on the development of Kant's ethics. Other philosophers have argued that Kant's parents and his teacher, Martin Knutzen, influenced his ethics. Those influenced by Kantian ethics include 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. German philosopher Arthur Schopenhauer argued that ethics should attempt to describe how people behave and criticised Kant for being prescriptive. Marcia Baron has defended the theory by arguing that duty does not diminish other motivations. The Catholic Church has criticised Kant's ethics as contradictory and regards Christian ethics as more compatible with virtue ethics.
The claim that all humans are due dignity and respect as autonomous agents means 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. Feminist philosophers have used Kantian ethics to condemn practices such as prostitution and pornography because they treat women as means. 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. 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.
Although all of Kant's work develops his ethical theory, it is most clearly defined in Groundwork of the Metaphysics of Morals, Critique of Practical Reason and Metaphysics of Morals. As part of the Enlightenment tradition, Kant based his ethical theory on the belief that reason should be used to determine how people ought to act. He did not attempt to prescribe specific action, but instructed that reason should be used to determine how to behave.
Good will and duty
In his combined works, Kant constructed the basis for an ethical law by the concept of duty. Kant began his ethical theory by arguing that the only virtue that can be unqualifiedly good is a good will. No other virtue has this status because every other virtue can be used to achieve immoral ends (the virtue of loyalty is not good if one is loyal to an evil person, for example). The good will is unique in that it is always good and maintains its moral value even when it fails to achieve its moral intentions. Kant regarded the good will as a single moral principle which freely chooses to use the other virtues for moral ends.
For Kant a good will is a broader conception than a will which acts from duty. A will which acts from duty is distinguishable as a will which overcomes hindrances in order to keep the moral law. A dutiful will is thus a special case of a good will which becomes visible in adverse conditions. Kant argues that only acts performed with regard to duty have moral worth. This is not to say that acts performed merely in accordance with duty are worthless (these still deserve approval and encouragement), but that special esteem is given to acts which are performed out of duty.
Kant's conception of duty does not entail that people perform their duties grudgingly. Although duty often constrains people and prompts them to act against their inclinations, it still comes from an agent's volition: they desire to keep the moral law. Thus, when an agent performs an action from duty it is because the rational incentives matter to them more than their opposing inclinations. Kant wished to move beyond the conception morality as externally imposed duties and present an ethics of autonomy, when rational agents freely recognise the claims reason makes upon them.
Perfect and imperfect duties
Applying the categorical imperative, duties arise because failure to fulfil them would either result in a contradiction in conception or in a contradiction in the will. The former are classified as perfect duties, the latter as imperfect. A perfect duty always holds true—there is a perfect duty to tell the truth, so we must never lie. An imperfect duty allows flexibility—beneficence is an imperfect duty because we are not obliged to be completely beneficent at all times, but may choose the times and places in which we are. Kant believed that perfect duties are more important than imperfect duties: if a conflict between duties arises, the perfect duty must be followed.
The primary formulation of Kant's ethics is the categorical imperative, from which he derived four further formulations. Kant made a distinction between categorical and hypothetical imperatives. A hypothetical imperative is one we must obey if we want to satisfy our desires: 'go to the doctor' is a hypothetical imperative because we are only obliged to obey it if we want to get well. A categorical imperative binds us regardless of our desires: everyone has a duty to not lie, regardless of circumstances and even if it is in our interest to do so. These imperatives are morally binding because they are based on reason, rather than contingent facts about an agent. Unlike hypothetical imperatives, which bind us insofar as we are part of a group or society which we owe duties to, we cannot opt out of the categorical imperative because we cannot opt out of being rational agents. We owe a duty to rationality by virtue of being rational agents; therefore, rational moral principles apply to all rational agents at all times.
Act only according to that maxim by which you can at the same time will that it should become a universal law.
When someone acts, it is according to a rule, or maxim. For Kant, an act is only permissible if one is willing for the maxim that allows the action to be a universal law by which everyone acts. Maxims fail this test if they produce either a contradiction in conception or a contradiction in the will when universalized. A contradiction in conception happens when, if a maxim were to be universalized, it ceases to make sense because the "...maxim would necessarily destroy itself as soon as it was made a universal law." For example, if the maxim 'It is permissible to break promises' was universalized, no one would trust any promises made, so the idea of a promise would become meaningless; the maxim would be self-contradictory because, when universalized, promises cease to be meaningful. The maxim is not moral because it is logically impossible to universalize—we could not conceive of a world where this maxim was universalized. A maxim can also be immoral if it creates a contradiction in the will when universalized. This does not mean a logical contradiction, but that universalizing the maxim leads to a state of affairs that no rational being would desire. For example, Driver argues that the maxim 'I will not give to charity' produces a contradiction in the will when universalized because a world where no one gives to charity would be undesirable for the person who acts by that maxim.
Kant believed that morality is the objective law of reason: just as objective physical laws necessitate physical actions (apples fall down because of gravity, for example), objective rational laws necessitate rational actions. He thus believed that a perfectly rational being must also be perfectly moral because a perfectly rational being subjectively finds it necessary to do what is rationally necessary. Because humans are not perfectly rational (they partly act by instinct), Kant believed that humans must conform their subjective will with objective rational laws, which he called conformity obligation. Kant argued that the objective law of reason is a priori, existing externally from rational being. Just as physical laws exist prior to physical beings, rational laws (morality) exist prior to rational beings. Therefore, according to Kant, rational morality is universal and cannot change depending on circumstance.
Humanity as an end in itself
Kant's second formulation of the Categorical Imperative is to treat humanity as an end in itself:
Act in such a way that you treat humanity, whether in your own person or in the person of another, always at the same time as an end and never simply as a means.
Kant argued that rational beings can never be treated merely as means to ends; they must always also be treated as ends themselves, requiring that their own reasoned motives must be equally respected. This derives from Kant's claim that reason motivates morality: it demands that we respect reason as a motive in all beings, including other people. A rational being cannot rationally consent to being used merely as a means to an end, so they must always be treated as an end. Kant justified this by arguing that moral obligation is a rational necessity: that which is rationally willed is morally right. Because all rational agents rationally will themselves to be an end and never merely a means, it is morally obligatory that they are treated as such. This does not mean that we can never treat a human as a means to an end, but that when we do, we also treat him as an end in himself.
Formula of autonomy
Kant's Formula of Autonomy expresses the idea that an agent is obliged to follow the Categorical Imperative because of their rational will, rather than any outside influence. Kant believed that any moral law motivated by the desire to fulfill some other interest would deny the Categorical Imperative, leading him to argue that the moral law must only arise from a rational will. This principle requires people to recognize the right of others to act autonomously and means that, as moral laws must be universalisable, what is required of one person is required of all.
Kingdom of Ends
Another formulation of Kant's Categorical Imperative is the Kingdom of Ends:
A rational being must always regard himself as giving laws either as member or as sovereign in a kingdom of ends which is rendered possible by the freedom of will.
This formulation requires that actions be considered as if their maxim is to provide a law for a hypothetical Kingdom of Ends. Accordingly, people have an obligation to act upon principles that a community of rational agents would accept as laws. In such a community, each individual would only accept maxims that can govern every member of the community without treating any member merely as a means to an end. Although the Kingdom of Ends is an ideal—the actions of other people and events of nature ensure that actions with good intentions sometimes result in harm—we are still required to act categorically, as legislators of this ideal kingdom.
Influences on Kantian ethics
Amanda & Khethani 2018 has suggested four strong influences on Kant's ethics. The first is the Lutheran sect Pietism, to which Kant's parents subscribed. Pietism emphasised honesty and moral living over doctrinal belief, more concerned with feeling than rationality. Kant believed that rationality is required, but that it should be concerned with morality and good will. Second is the political philosopher Jean-Jacques Rousseau, whose work, The Social Contract, influenced Kant's view on the fundamental worth of human beings. Pojman also cites contemporary ethical debates as influential to the development of Kant's ethics. Kant favoured rationalism over empiricism, which meant he viewed morality as a form of knowledge, rather than something based on human desire. Natural law (the belief that the moral law is determined by nature) and intuitionism (the belief that humans have intuitive awareness of objective moral truths) were, according to Pojman, also influential for Kant.
Biographer of Kant, Manfred Kuhn, suggested that the values Kant's parents held, of "hard work, honesty, cleanliness, and independence”, set him an example and influenced him more than their Pietism did. In the Stanford Encyclopedia of Philosophy, Michael Rohlf suggests that Kant was influenced by his teacher, Martin Knutzen, himself influenced by the work of Christian Wolff and John Locke, and who introduced Kant to the work of English physicist Isaac Newton.
Significance of Kantian ethics
Influenced by Kantian ethics
German philosopher Jürgen Habermas has proposed a theory of discourse ethics that he claims is a descendant of Kantian ethics. He proposes that action should be based on communication between those involved, in which their interests and intentions are discussed so they can be understood by all. Rejecting any form of coercion or manipulation, Habermas believes that agreement between the parties is crucial for a moral decision to be reached. Like Kantian ethics, discourse ethics is a cognitive ethical theory, in that it supposes that truth and falsity can be attributed to ethical propositions. It also formulates a rule by which ethical actions can be determined and proposes that ethical actions should be universalisable, in a similar way to Kant's ethics.
Habermas argues that his ethical theory is an improvement on Kant's ethics. He rejects the dualistic framework of Kant's ethics. Kant distinguished between the phenomena world, which can be sensed and experienced by humans, and the noumena, or spiritual world, which is inaccessible to humans. This dichotomy was necessary for Kant because it could explain the autonomy of a human agent: although a human is bound in the phenomenal world, their actions are free in the intelligible world. For Habermas, morality arises from discourse, which is made necessary by their rationality and needs, rather than their freedom.
The social contract theory of political philosopher John Rawls, developed in his work A Theory of Justice, was influenced by Kant's ethics. Rawls argued that a just society would be fair. To achieve this fairness, he proposed a hypothetical moment prior to the existence of a society, at which the society is ordered: this is the original position. This should take place from behind a veil of ignorance, where no one knows what their own position in society will be, preventing people from being biased by their own interests and ensuring a fair result. Rawls' theory of justice rests on the belief that individuals are free, equal, and moral; he regarded all human beings as possessing some degree of reasonableness and rationality, which he saw as the constituents of morality and entitling their possessors to equal justice. Rawls dismissed much of Kant's dualism, arguing that the structure of Kantian ethics, once reformulated, is clearer without it—he described this as one of the goals of A Theory of Justice.
French psychoanalyst Jacques Lacan linked psychoanalysis with Kantian ethics in his works The Ethics of Psychoanalysis and Kant avec Sade and compared Kant with the Marquis de Sade. Lacan argued that Sade's maxim of jouissance—the pursuit of sexual pleasure or enjoyment—is morally acceptable by Kant's criteria because it can be universalised. He proposed that, while Kant presented human freedom as critical to the moral law, Sade further argued that human freedom is only fully realised through the maxim of jouissance.
Contemporary Kantian ethicists
Philosopher Onora O'Neill, who studied under John Rawls at ￼Harvard University, is a contemporary Kantian ethicist who supports a Kantian approach to issues of social justice. O'Neill argues that a successful Kantian account of social justice must not rely on any unwarranted idealizations or assumption. She notes that philosophers have previously charged Kant with idealizing humans as autonomous beings, without any social context or life goals, though maintains that Kant's ethics can be read without such an idealization. O'Neill prefers Kant's conception of reason as practical and available to be used by humans, rather than as principles attached to every human being. Conceiving of reason as a tool to make decisions with means that the only thing able to restrain the principles we adopt is that they could be adopted by all. If we cannot will that everyone adopts a certain principle, then we cannot give them reasons to adopt it. To use reason, and to reason with other people, we must reject those principles that cannot be universally adopted. In this way, O'Neill reached Kant's formulation of universalisability without adopting an idealistic view of human autonomy. This model of universalisability does not require that we adopt all universalisable principles, but merely prohibits us from adopting those that are not.
From this model of Kantian ethics, O'Neill begins to develop a theory of justice. She argues that the rejection of certain principles, such as deception and coercion, provides a starting point for basic conceptions of justice, which she argues are more determinate for human beings that the more abstract principles of equality or liberty. Nevertheless, she concedes that these principles may seem to be excessively demanding: there are many actions and institutions that do rely on non-universalisable principles, such as injury.
In his paper The Schizophrenia of Modern Ethical Theories, philosopher Michael Stocker challenges Kantian ethics (and all modern ethical theories) by arguing that actions from duty lack certain moral value. He gives the example of Smith, who visits his friend in hospital out of duty, rather than because of the friendship; he argues that this visit seems morally lacking because it is motivated by the wrong thing. Marcia Baron has attempted to defend Kantian ethics on this point. After presenting a number of reasons that we might find acting out of duty objectionable, she argues that these problems only arise when people misconstrue what their duty is. Acting out of duty is not intrinsically wrong, but immoral consequences can occur when people misunderstand what they are duty-bound to do. Duty need not be seen as cold and impersonal: one may have a duty to cultivate their character or improve their personal relationships. Baron further argues that duty should be construed as a secondary motive—that is, a motive that regulates and sets conditions on what may be done, rather than prompt specific actions. She argues that, seen this way, duty neither reveals a deficiency in one's natural inclinations to act, nor undermines the motives and feelings that are essential to friendship. For Baron, being governed by duty does not mean that duty is always the primary motivation to act; rather, it entails that considerations of duty are always action-guiding. A responsible moral agent should take an interest in moral questions, such as questions of character. These should guide moral agents to act from duty.
Critics of Kantian ethics
G. W. F Hegel
German philosopher G. W. F. Hegel presented two main criticisms of Kantian ethics. He first argued that Kantian ethics provides no specific information about what people should do because Kant's moral law is solely a principle of non-contradiction. He argued that Kant's ethics lack any content and so cannot constitute a supreme principle of morality. To illustrate this point, Hegel and his followers have presented a number of cases in which the Formula of Universal Law either provides no meaningful answer or gives an obviously wrong answer. Hegel used Kant's example of being trusted with another man's money to argue that Kant's Formula of Universal Law cannot determine whether a social system of property is a morally good thing, because either answer can entail contradictions. He also used the example of helping the poor: if everyone helped the poor, there would be no poor left to help, so beneficence would be impossible if universalised, making it immoral according to Kant's model. Hegel's second criticism was that Kant's ethics forces humans into an internal conflict between reason and desire. For Hegel, it is unnatural for humans to suppress their desire and subordinate it to reason. This means that, by not addressing the tension between self-interest and morality, Kant's ethics cannot give humans any reason to be moral.
German philosopher Arthur Schopenhauer criticised Kant's belief that ethics should concern what ought to be done, insisting that the scope of ethics should be to attempt to explain and interpret what actually happens. Whereas Kant presented an idealized version of what ought to be done in a perfect world, Schopenhauer argued that ethics should instead be practical and arrive at conclusions that could work in the real world, capable of being presented as a solution to the world's problems. Schopenhauer drew a parallel with aesthetics, arguing that in both cases prescriptive rules are not the most important part of the discipline. Because he believed that virtue cannot be taught—a person is either virtuous or is not—he cast the proper place of morality as restraining and guiding people's behavior, rather than presenting unattainable universal laws.
Philosopher Friedrich Nietzsche criticised all contemporary moral systems, with a special focus on Christian and Kantian ethics. He argued that all modern ethical systems share two problematic characteristics: first, they make a metaphysical claim about the nature of humanity, which must be accepted for the system to have any normative force; and second, the system benefits the interests of certain people, often over those of others. Although Nietzsche's primary objection is not that metaphysical claims about humanity are untenable (he also objected to ethical theories that do not make such claims), his two main targets—Kantianism and Christianity—do make metaphysical claims, which therefore feature prominently in Nietzsche's criticism.
Nietzsche rejected fundamental components of Kant's ethics, particularly his argument that morality, God, and immorality, can be shown through reason. Nietzsche cast suspicion on the use of moral intuition, which Kant used as the foundation of his morality, arguing that it has no normative force in ethics. He further attempted to undermine key concepts in Kant's moral psychology, such as the will and pure reason. Like Kant, Nietzsche developed a concept of autonomy; however, he rejected Kant's idea that valuing our own autonomy requires us to respect the autonomy of others. A naturalist reading of Nietzsche's moral psychology stands contrary to Kant's conception of reason and desire. Under the Kantian model, reason is a fundamentally different motive to desire because it has the capacity to stand back from a situation and make an independent decision. Nietzsche conceives of the self as a social structure of all our different drives and motivations; thus, when it seems that our intellect has made a decision against our drives, it is actually just an alternative drive taking dominance over another. This is in direct contrast with Kant's view of the intellect as opposed to instinct; instead, it is just another instinct. There is thus no self-capable of standing back and making a decision; the decision the self-makes is simply determined by the strongest drive. Kantian commentators have argued that Nietzsche's practical philosophy requires the existence of a self-capable of standing back in the Kantian sense. For an individual to create values of their own, which is a key idea in Nietzsche's philosophy, they must be able to conceive of themselves as a unified agent. Even if the agent is influenced by their drives, he must regard them as his own, which undermines Nietzsche's conception of autonomy.
John Stuart Mill
Utilitarian philosopher John Stuart Mill criticised Kant for not realizing that moral laws are justified by a moral intuition based on utilitarian principles (that the greatest good for the greatest number ought to be sought). Mill argued that Kant's ethics could not explain why certain actions are wrong without appealing to utilitarianism. As basis for morality, Mill believed that his principle of utility has a stronger intuitive grounding than Kant's reliance on reason, and can better explain why certain actions are right or wrong.
Virtue ethics is a form of ethical theory which emphasizes the character of an agent, rather than specific acts; many of its proponents have criticised Kant's deontological approach to ethics. Elizabeth Anscombe criticised modern ethical theories, including Kantian ethics, for their obsession with law and obligation. As well as arguing that theories which rely on a universal moral law are too rigid, Anscombe suggested that, because a moral law implies a moral lawgiver, they are irrelevant in modern secular society. In his work After Virtue, Alasdair MacIntyre criticises Kant's formulation of universalisability, arguing that various trivial and immoral maxims can pass the test, such as "Keep all your promises throughout your entire life except one". He further challenges Kant's formulation of humanity as an end in itself by arguing that Kant provided no reason to treat others as means: the maxim "Let everyone except me be treated as a means", though seemingly immoral, can be universalized. Bernard Williams argues that, by abstracting persons from character, Kant misrepresents persons and morality and Philippa Foot identified Kant as one of a select group of philosophers responsible for the neglect of virtue by analytic philosophy.
Roman Catholic priest Servais Pinckaers regarded Christian ethics as closer to the virtue ethics of Aristotle than Kant's ethics. He presented virtue ethics as freedom for excellence, which regards freedom as acting in accordance with nature to develop one's virtues. Initially, this requires following rules—but the intention is that the agent develop virtuously, and regard acting morally as a joy. This is in contrast with freedom of indifference, which Pinckaers attributes to William Ockham and likens to Kant. On this view, freedom is set against nature: free actions are those not determined by passions or emotions. There is no development or progress in an agent's virtue, merely the forming of habit. This is closer to Kant's view of ethics, because Kant's conception of autonomy requires that an agent is not merely guided by their emotions, and is set in contrast with Pinckaer's conception of Christian ethics.
A number of philosophers, including Elizabeth Anscombe , Jean Bethke Elshtain , Servais Pinckaers, Iris Murdoch, and the Catholic Encyclopedia, have all suggested that the Kantian conception of ethics rooted in autonomy is contradictory in its dual contention that humans are co-legislators of morality and that morality is a priori. They argue that if something is universally a priori (i.e., existing unchangingly prior to experience), then it cannot also be in part dependent upon humans, who have not always existed. On the other hand, if humans truly do legislate morality, then they are not bound by it objectively, because they are always free to change it.
This objection seems to rest on a misunderstanding of Kant's views since Kant argued that morality is dependent upon the concept of a rational will (and the related concept of a categorical imperative: an imperative which any rational being must necessarily will for itself). It is not based on contingent features of any being's will, nor upon human wills in particular, so there is no sense in which Kant makes ethics "dependent" upon anything which has not always existed. Furthermore, the sense in which our wills are subject to the law is precisely that if our wills are rational, we must will in a lawlike fashion; that is, we must will according to moral judgments we apply to all rational beings, including ourselves. This is more easily understood by parsing the term "autonomy" into its Greek roots: auto (self) + nomos (rule or law). That is, an autonomous will, according to Kant, is not merely one which follows its own will, but whose will is lawful-that is, conforming to the principle of universalizability, which Kant also identifies with reason. Ironically, in another passage, willing according to immutable reason is precisely the kind of capacity Elshtain ascribes to God as the basis of his moral authority, and she commands this over an inferior voluntarist version of divine command theory, which would make both morality and God's will contingent. Kant's theory is a version of the first rather than the second view of autonomy, so neither God nor any human authority, including contingent human institutions, play any unique authoritative role in his moral theory. Kant and Elshtain, that is, both agree God has no choice but to conform his will to the immutable facts of reason, including moral truths; humans do have such a choice, but otherwise their relationship to morality is the same as that of God's: they can recognize moral facts, but do not determine their content through contingent acts of will.
Kant believed that the shared ability of humans to reason should be the basis of morality, and that it is the ability to reason that makes humans morally significant. He, therefore, believed that all humans should have the right to common dignity and respect. Margaret Eaton argues that, according to Kant's ethics, a medical professional must be happy for their own practices to be used by and on anyone, even if they were the patient themselves. For example, a researcher who wished to perform tests on patients without their knowledge must be happy for all researchers to do so. She also argues that Kant's requirement of autonomy would mean that a patient must be able to make a fully informed decision about treatment, making it immoral to perform tests on unknowing patients. Medical research should be motivated out of respect for the patient, so they must be informed of all facts, even if this would be likely to dissuade the patient. Jeremy Sugarman has argued that Kant's formulation of autonomy requires that patients are never used merely for the benefit of society, but are always treated as rational people with their own goals. Aaron Hinkley notes that a Kantian account of autonomy requires respect for choices that are arrived at rationally, not for choices which are arrived at by idiosyncratic or non-rational means. He argues that there may be some difference between what a purely rational agent would choose and what a patient actually chooses, the difference being the result of non-rational idiosyncrasies. Although a Kantian physician ought not to lie to or coerce a patient, Hinkley suggests that some form of paternalism - such as through withholding information which may prompt a non-rational response - could be acceptable.
In her work How Kantian Ethics Should Treat Pregnancy and Abortion, Susan Feldman argues that abortion should be defended according to Kantian ethics. She proposed that a woman should be treated as a dignified autonomous person, with control over their body, as Kant suggested. She believes that the free choice of women would be paramount in Kantian ethics, requiring abortion to be the mother's decision. Dean Harris has noted that, if Kantian ethics is to be used in the discussion of abortion, it must be decided whether a fetus is an autonomous person. Kantian ethicist Carl Cohen argues that the potential to be rational or participation in a generally rational species is the relevant distinction between humans and inanimate objects or irrational animals. Cohen believes that even when humans are not rational because of age (such as babies or fetuses) or mental disability, agents are still morally obligated to treat them as an ends in themselves, equivalent to a rational adult such as a mother seeking an abortion.
Kant viewed humans as being subject to the animalistic desires of self-preservation, species-preservation, and the preservation of enjoyment. He argued that humans have a duty to avoid maxims that harm or degrade themselves, including suicide, sexual degradation, and drunkenness. This led Kant to regard sexual intercourse as degrading because it reduces humans to an object of pleasure. He admitted sex only within marriage, which he regarded as "a merely animal union". He believed that masturbation is worse than suicide, reducing a person's status to below that of an animal; he argued that rape should be punished with castration and that bestiality requires expulsion from society. Feminist philosopher Catharine MacKinnon has argued that many contemporary practices would be deemed immoral by Kant's standards because they dehumanize women. Sexual harassment, prostitution and pornography, she argues, objectify women and do not meet Kant's standard of human autonomy. Commercial sex has been criticised for turning both parties into objects (and thus using them as a means to an end); mutual consent is problematic because in consenting, people choose to objectify themselves. Alan Soble has noted that more liberal Kantian ethicists believe that, depending on other contextual factors, the consent of women can vindicate their participation in pornography and prostitution.
Because Kant viewed rationality as the basis for being a moral patient—one due moral consideration—he believed that animals have no moral rights. Animals, according to Kant, are not rational, thus one cannot behave immorally towards them. Although he did not believe we have any duties towards animals, Kant did believe being cruel to them was wrong because our behaviour might influence our attitudes toward human beings: if we become accustomed to harming animals, then we are more likely to see harming humans as acceptable.
Ethicist Tom Regan rejected Kant's assessment of the moral worth of animals on three main points: First, he rejected Kant's claim that animals are not self-conscious. He then challenged Kant's claim that animals have no intrinsic moral worth because they cannot make a moral judgment. Regan argued that, if a being's moral worth is determined by its ability to make a moral judgment, then we must regard humans who are incapable of moral thought as being equally undue moral consideration. Regan finally argued that Kant's assertion that animals exist merely as a means to an end is unsupported; the fact that animals have a life that can go well or badly suggests that, like humans, they have their own ends.
However, Christine Korsgaard reinterpreted Kantian theory to establish that animal rights are implied by his moral principles.
Kant believed that the Categorical Imperative provides us with the maxim that we ought not to lie in any circumstances, even if we are trying to bring about good consequences, such as lying to a murderer to prevent them from finding their intended victim. Kant argued that, because we cannot fully know what the consequences of any action will be, the result might be unexpectedly harmful. Therefore, we ought to act to avoid the known wrong—lying—rather than to avoid a potential wrong. If there are harmful consequences, we are blameless because we acted according to our duty. Driver argues that this might not be a problem if we choose to formulate our maxims differently: the maxim 'I will lie to save an innocent life' can be universalized. However, this new maxim may still treat the murderer as a means to an end, which we have a duty to avoid doing. Thus we may still be required to tell the truth to the murderer in Kant's example.
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