Having dealt with Moore's Naturalistic Fallacy yesterday it is appropriate to deal with Hume's Is-Ought problem with which is is often confused. However unlike Moore's argument there is nothing false about Hume's argument, it just does not lead to problems as an ethical naturalist, but it is still fatal to many subjective systems that falsely claim they are objective such as Divine Command and Randian Objectivism.
The Is-Ought Problem
There is no better reference that what Hume himself said as an afterthought in his "A 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 all 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,that 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.and I wholeheartedly agree with this. To put it simply you can't conclude with an ought given only is premises.
You can derive an 'ought' from an 'ought'
However there is nothing wrong if there is an ought in the premises to draw a further ought in the conclusion. As John Searle says in "How to derive an 'ought' from an 'is' "
Put in more contemporary terminology, no set of descriptive statements can entail an evaluative statement without the addition of at least one evaluative premise.However unlike Searle we do not need to attempt to do this, indeed I think Searle was mistaken in his argument (see the link above) but that is outside the topic of this post (I will examine it in the future, analyzed in terms of Desire Fulfillment).
Another reference, which incidentally demolished Randian Objectivists arguments over deriving ought from is - some of them say, fallaciously, that all 'is's are' oughts' (but more on this in a future post) - in "Ayn Rand and the Is-Ought Problem" by Patrick M.O'Neil:
It should be noted that, although many philosophers speak as if the is-ought problem were a problem in logic, the question does not belong to the domain of the science of logic at all. The problem can, of course, be expressed in logical terms, for the impossibility of deriving an "ought" from an "is" can be rendered as the inability to reach prescriptive conclusions from descriptive premises. Despite the use of the terminology of logic in setting up the problem, however, there exists no necessity from the point of view of the science of logic alone why one should expect to be able to derive an "ought" from an "is." No paradox or antinomy results from this inability, and prescriptive conclusions can be produced by the expedient of utilizing a prescriptive premise."[my emphasis]And this is the approach here.
As Tom Clarke from the Center for Naturalism says
Clearly normativity is natural, in that ethical rules or norms are based in human biological needs and innate psychological dispositions, modulated by culture.So what are these natural norms ? An agent acts to fulfill the more and stronger of their desires, which includes all the biological needs, innate psychological dispositions and cultural modulations, given these actions - whether they achieve fulfillment or not - the desire is the "natural norm" (or reason to act), if they do not have their desire they have no reason to act and would not so act. So an ought is nothing other than a reason to act, since for agent to ought to F, is for them to have a reason to act to F - and here, in Desire Fulfillment, a reason to act is nothing other than a desire.
A theory of value is a theory of prescription
So in developing a theory of value - the desire fulfillment theory of value - we have also developed a theory of prescription - a theory of reasons to act - and the theory of value describes these prescriptions - namely the desires and how they are fulfilled (or not).
So in analyzing any situation, once one has identified the relevant desires (the reasons to act), significant desires (the materially affected desires) , the states of affairs and their relations, one can evaluated and draw conclusions this in terms of oughts - in terms of desire fulfilling desires and desire thwarting desires. Since one is starting with oughts (desires) in the premise, there is no issue with ending with such oughts in the conclusion.
So Hume was quite correct, one cannot derive 'ought' from 'is' and any philosophy which does this is a "vulgar philosophy". Desire Fulfillment is no such philosophy since it does not derive 'ought' from 'is'.