Here are some very specific critiques of Kant's attempt to base morality on reason, however general critiques of Kant would work too. For example, the clashing duties criticism suggests that ethics cannot be bassed on reason figuring out our duties, for example.
Kant’s attempt to base morality on reason.
Kant was part of the intellectual movement called the enlightenment, which was in part a reaction to religious warfare which had been destructive in Europe and many wanted to find a way to prevent it.
Whereas natural law thought reason could discover a natural moral law in our nature, Kant thought reason could discover a moral law in reason itself. Kant claims that what it means to be human is to have be a ‘rational agent’, which means to have reason, be able to make choices and have goals (ends). Our reason also tells us that all humans are rational agents and that therefore we are all equal.
It is unreasonable to act contrary to what our reason tells us is the case. If I did an action that couldn’t be done by everyone, then I would have to think that I was somehow special or better than others. However, reason tells us we are all equal. So, reason tells us that we should only act on principles that can be followed by everyone.
An action could only in accordance with the universal moral law if it could be done by all people in all situations. Kant called this a categorical imperative; something we should always do (you should do X). A hypothetical imperative is a moral action that a rational will adopts for reasons other than duty (you should do X if you want Y). As rational beings we may adopt ends that are not categorical, which makes them hypothetical. However, true morality and duty, the universal moral law, cannot depend on our desires and so must be categorical.
Duty & The Good Will. For Kant, a Good will is one which has the right intention when performing moral actions. Once we have used our reason to figure out our duty, we should then just do it out of a sense of duty because it is our duty. We should leave out personal feelings/desires and just do ‘duty for duty’s sake’. For example, if it is our duty to give money to charity, we should do it because it is our duty, not because we want to or because we feel empathy. The only morally good motivation for doing an action is out of a sense of duty.
Bernard Williams claims it is inhuman and ethically wrong to suggest that moral judgement should be free from emotion and an ethic like Kant’s which recommends it is therefore immoral. For example, giving money to charity because you feel empathy for suffering people seems like a moral act, but Kant would regard it as non-moral.
Kant would respond by arguing that something is either right or wrong regardless of how a person might feel about it. Those who think it morally good to give money to charity out of empathy are actually committing themselves to the claim that the goodness of the act consists in their feelings of empathy, at least in part. If they asked themselves why it was good to give money to suffering people, however, satisfying the empathetic feelings of the giver would generally not be considered a reason. The deservedness of the receiver of charity is not thought by anyone to depend on the presence of feelings of empathy on the part of the giver. Therefore, those who think it morally good to give to charity out of empathy should recognize, Kant would argue, that the goodness of their act does not depend on their feelings. Acting out of feelings is therefore failing to act morally.
Arguably it is actually impossible in practice to act without any influence of emotion on your moral motivation. So, Kant’s ethics may be good in abstract theory, but don’t work in practice given the kind of emotional beings that we in fact are.. This is what Hume argues.
Hume’s meta-ethics was greatly disliked by Kant and motivated Kant to create his own ethical theory. Kant thinks ethics can be based on reason and that we can and should remove emotion as a motivation for moral decision making. However, Hume claims that moral judgements being motivating means they must involve desire, which is an emotion or sentiment. It’s not enough merely to reason that we should do something because why would we care that we should do what we should do unless we had a desire to do what we should do? Hume claims that we just are the sort of being which cannot help but require desire in order to be motivated to do actions, which means Kant’s ideal of the good will is an impossible ideal.
P1 – moral judgements are intrinsically motivating.
P2 – Reason is not intrinsically motivating.
C1 – Therefore, moral judgements cannot be derived from reason alone.
Rational agents can put their emotion aside. The idea that reason and emotion are in conflict goes back to Plato, who saw human reason as aimed higher than the world at intellectual abstract ideas, in conflict with the body which anchored reason in the mere physical world with animalistic feelings. Kant too clearly thinks something like this and suggests that, as rational agents, we can and should try to separate our reason from emotional influence.
However, Hume claimed that “reason is, and ought only to be, the slave of the passions”. There are everyday examples which illustrate this. When someone criticises your deeply held personal belief, your mind instantly starts thinking of defences. If it cannot think of anything, it starts getting angry and projecting negative psychological motivations into the critic. This looks like your mind has pre-conceived feelings and the role of reason and rationality is merely to provide ad hoc rationalisations to serve our prejudices. Our mind is more like a lawyer than a scientist.
It is our culture which determines our emotional feelings. Kant’s views on sexual ethics are an excellent example of how his supposed reasoned moral views were really just reflections of and rationalisations for his culture’s views:
Homosexuality is an "unmentionable vice" so wrong that "there are no limitations whatsoever that can save [it] from being repudiated completely" (p. 277).
A child that comes into the world apart from marriage is born outside the law (for the law is marriage) and therefore outside the protection of the law. It has, as it were, stolen into the commonwealth (like contraband merchandise), so that the commonwealth can ignore its existence (since it rightly should not have come to exist in this way), and can therefore also ignore its annihilation (p. 336).
Regarding Kant, there is a difference between the logic of his theory which arguably leads to fairly liberal views, and his own personal views which were rigidly traditional and conservative. Some argue that this actually demonstrates a serious critique of Kant’s ethics. Kant imagined that ethics could be based on reason, yet when it came to the practical implementation of his ethics to sexual issues, he was just as much a product of his culture as the most unthinking and unreasonable person in it.