In a series of books, esp. The Language of Morals, Freedom and Reason, and Moral Thinking, Hare gave shape to a theory that he called Universal prescriptivism. According to it, moral terms such as 'good', 'ought' and 'right' have two logical (i.e. semantic) properties: universalizability and prescriptiviy. By the former, moral judgments must identify the situation that they describe by a finite set of universal terms (this excludes proper names, but not definite descriptions). By the latter, moral agents must do those acts that they consider morally obligatory whenever they are physically and psychologically able to do them. Hare noted that the combination of the two properties leads to a certain form of consequentialism, namely, preference utilitarianism.
To see how this works, let's consider the following imaginary situation. Suppose that you need a big sum of money and you ask your friend to lend it to you. He refuses. So you claim that it is wrong for him to do so. 'Wrong' is a moral term, so, according to Hare, you must abide by its logical properties. The first property, universalizability, demands that you give a description of the situation using only universal terms. So you say:
Whenever I ask a friend for a big sum of money, it is wrong for her to refuse to give it to me.But this violates the universalizability requirement, insofar as the description contains the terms 'I' and 'me', which do not designate a universal property, but denote an individual instead. So you try again:
Whenver someone asks a friend for a big sum of money, it is wrong for her to refuse the request.This new description satisfies the universalizability requirement, as all its terms are universal. This requirement aroused from the first property of the moral terms; let us now examine that which arises from its second, i.e., prescriptiviy.
We must, then, see whether you are willing to act according to your initial judgment. At first sight, it would seem that this does not apply to you: if you consider it wrong for your friend to refuse to lend you a big sum of money, it is your friend, not you, the one who should be acting accordingly. However -and here is where the two properties combine and the philosophically interesting results appear-, universalizability allows for the same judgment to be made irrespective of your position in the situation. In other words, as you had to deprive the description of its particular (i.e. non-universal) terms, it is now impossible for you to exclude yourself from the possibility of being in the situation that your friend was. So, by universalizability, if you happened to be, not the one asking, but the one asked the money, the same moral judgment would still apply; and, by prescriptiviy, you would have to act accordingly. If you were not prepared to do what the judgment asked you to do, i.e., lend the big sum of money, then you would be violating one of the requirements of morality, and in fact you wouldn't be uttering a moral judgment at all. To re-enter the moral discourse, you would have to modify your original judgment so that, once universalized, you would still be able to act in the way it would ask you to act. By a series of (universal) conjectures and (prescriptive) refutations -akin to philosopher Karl Popper's falsificationism (see Freedom and Reason, chapter 4)- you would eventually arrive at the right moral judgment, which would be the one you would prefer in all the possible situations. Universal prescriptivism, thus, leads to preference utilitarianism. And so, according to Hare, does kantianism: to demand, as Kant's first formulation of the categorical imperative does, that you could will that your maxim be a universal law, is to ask the moral agent to prescribe that judgment that she could accept were she in any of the positions involved --which of course is exactly Hare's point.
Hare taught at Oxford, and, after retiring from his post there, at the University of Florida. Among his students, there were some who would later become famous, such as Bernard Williams and, more notably, Peter Singer, who has explicitly adopted many elements of Hare's thought.
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