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In meta-ethics, the is-ought problem was raised by David Hume (Scottish philosopher and historian, 1711-1776), who noted that many writers make claims about what ought to be on the basis of statements about what is. But there seems to be a big difference between descriptive statements (about what is) and prescriptive statements (about what ought to be).
Hume discusses the problem in book III, part I, section I of his Treatise of Human Nature :
- In every system of morality, which I have hitherto met with, I have always remark'd, that the author proceeds for some time in the ordinary ways of reasoning, and establishes the being of a God, or makes observations concerning human affairs; when of a sudden I am surpriz'd to find, that instead of the usual copulations of propositions, is, and is not, I meet with no proposition that is not connected with an ought, or an ought not. This change is imperceptible; but is however, of the last consequence. For as this ought, or ought not, expresses some new relation or affirmation, 'tis necessary that it shou'd be observ'd and explain'd; and at the same time that a reason should be given; for what seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it.
Hume then calls for writers to be on their guard against such reasoning, if they cannot give an explanation of how the ought-statements are supposed to follow from the is-statements. But how exactly can you derive an "ought" from an "is"? That question, prompted by Hume's small paragraph, has become one of the central questions of ethical theory, and Hume is usually assigned the position that such a derivation is impossible.
A similar (though distinct) view is defended by G. E. Moore's 'open question argument', intended to refute any identification of moral properties with natural properties — the so-called 'naturalistic fallacy'. And, like the naturalistic fallacy (which is often misunderstood to involve the arguably fallacious inference from 'this is (un)natural' to 'this is (im)moral'), the is-ought problem has been misunderstood as related to a less deep, but more common, issue. Namely, many people think the is-ought problem is related to the arguably true claim that, just because something is the case, that doesn't mean that it ought to be the case. On this misunderstanding, Hume was arguing against those complacent moralists who hold that the world is just fine as it stands, and needs no improvement. This is, of course, not Hume's point — he meant to challenge the transition from any set of descriptive claims to any prescriptive claim. This is a very general and very deep challenge to any descriptive account of moral thought, a challenge that is in keeping with Hume's anti-rationalist bent as a moral theorist.
Many people find Hume's thesis compelling, and see no hope for grounding moral statements in purely descriptive ones. A handful of arguments have been proposed which claim to show that an "ought" can actually be derived from an "is". John Searle devised one such argument. Basically, it tries to show that the act of making a promise places one under an obligation by definition, and that such an obligation amounts to an "ought". This view is still widely debated, and to answer criticisms, Searle has further developed the concept of institutional facts -- for example that a certain building is a bank, and that certain paper is money would be facts that seem to depend upon general recognition of those institutions and their value. See Social construction.
Consequences of is-ought problem
The apparent gap between "is" statements and "ought" statements, when combined with Hume's fork — the idea that all items of knowledge either are based on logic and definitions or are based on observation — renders "ought" statements of dubious validity. Since "ought" statements do not seem to be known in either of the two ways mentioned, it would seem that there can be no moral knowledge. Two responses to this are moral skepticism and non-cognitivism.
The answer of the those who believe in actual moral knowledge depends upon a few presuppositions. One has to do with the definition of reality that descriptive truths are said to correspond to. Another has to do with the existence of indefinables.
By reality, an effective moral cognitivist response assumes it means those things actually existing independent of the mind, rather than those representations of things independent of the mind in the mind that we call knowledge, or of wishes entertained that things might be otherwise. In that more actual sense of the meaning of reality, an effective moral cognitivist response can agree that the truth of "is" statements are ultimately based on their correspondence to reality (both in the realm of actuality and the ideal) while "ought" statements are not.
By indefinables, this refers to concepts so global that they cannot be defined; rather, in a sense, they themselves define our reality and our ideas. Their meanings cannot be stated in a true definition, but their meanings can be referred to instead by being placed with their incomplete definitions in self-evident statements, the truth of which can be tested by whether or not it is impossible to think the opposite without a contradiction.
An example of this is of finite parts and wholes; they cannot be defined without reference to each other and thus with some amount of circularity, but we can make the self-evident statement that "the whole is greater than any of its parts", and thus establish a meaning particular to the two concepts.
These two notions being granted, it can be said that statements of "ought" are measured by their prescriptive truth, just as statements of "is" are measured by their descriptive truth; and the descriptive truth of an "is" judgment is defined by its correspondence to reality (actual or in the mind), while the prescriptive truth of an "ought" judgment is defined according to a more limited scope--its correspondence to right desire (conceivable in the mind and able to be found in the rational appetite, but not in the more actual reality of things independent of the mind or rational appetite).
To some, this may immediately suggest the question: "How can we know what is right desire if it is already admitted that it is not based on the more actual reality of things independent of the mind?" The beginning of the answer is found when we consider that the concepts "good", "bad", "right" and "wrong" are indefinables. Thus right desire cannot be defined properly, but a way to refer to its meaning may be found through a self-evident prescriptive truth.
That self-evident truth which the moral cognitivist claims to exist upon which all other prescriptive truths are ultimately based is: One ought to desire what is really good for one and nothing else. The terms "Good" and "right desire" cannot be defined apart from each other, and thus their definitions would contain some degree of circularity, but the stated self-evident truth indicates a meaning particular to the ideas sought to be understood, and it is (the moral cognitivist claims) impossible to think the opposite without a contradiction. Thus combined with other descriptive truths of what is good, a valid body of knowledge of right desire is generated.
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