Hume observed that every system of morality he encountered started out with "is-statements" and then at some point, without explanation, switches to "ought-statements." He finds the relation expressed by "ought-statements" so different from that expressed by "is-statements" that he writes that it "seems altogether inconceivable, how this new relation can be a deduction from others, which are entirely different from it" (Hume, III-I-I). Many have taken Hume to mean that there is an unbridgeable gulf between "ought-statements" and "is-statements." A weaker claim would be that the relation between ought-statements and is-statements cannot be one of strict entailment, while not denying that there may be some weaker connection. Alasdair MacIntyre, in his article "Hume on 'is' and 'ought'," argues that Hume is not making the strong claim of an unbridgeable gap but rather insisting that the relationship should be explained.

G. E. Moore called the attempt to define the good in terms of something natural or metaphysical "the Naturalistic Fallacy." A key component of Moore's argument was that when one tried to define "good" in these terms, one could always intelligibly ask of the proposed definition, "But is it good?" This is called the "open question argument."

John Searle suggests that lurking behind the idea that "ought" cannot be derived from "is" is what he calls the "classical empiricist picture" (Cahn, p.414), which involves the idea that "is-statements" make descriptive claims about the world and thus have truth values which are "objectively decidable." "Ought-statements," on the other hand, express evaluations and so do not have objective truth values. Searle argues that this picture is wrong. He suggests that it ignores a certain kind of descriptive statement. Or, said another way, it does not make the distinction between two kinds of descriptive statements, between brute facts on the one hand and institutional facts on the other. Institutional facts, by their nature, in combination with brute facts can yield obligations. Institutional facts are composed of constitutive rules, that is, rules that create a form of activity that cannot exist independently of those rules. For example, chess cannot exist apart from the rules that constitute it. (This is opposed to regulative rules, such as "the rules of the road" that regulate an activity, driving, which could exist without such rules.)

Now, given an institution, and the rules that constitute it, together with brute facts that count as actions falling under the domain of those rules, obligations issue forth. This is because some constitutive rules entail obligations. For example a person standing in a chalk box in front of a pentagon shaped "plate" swinging a stick at a ball and missing, counts as a strike. And, after the third strike, the batter is obligated to leave the batting area.

The "oughts" that perplexed Hume are ones that appeared in "systems of morality." But can institutional facts suffice for a proof of a moral obligation, or only some other kind? It seems clearly possible to have an immoral institution with constitutive rules that entail immoral imperatives. Suppose I am playing a sadistic version of Russian roulette where instead of pointing the gun at one's own head one points it at the head of the person to one's left. As long as those involved know the rules of this game and collectively decide to play it, it is no less an institution than baseball. And when it comes my turn, I have the same sort of obligation to aim at my neighbor's head and pull the trigger as I would have to leave the batting area after striking out. A version of Moore's open question argument is applicable here. Whatever obligation is derived from an institutional fact, it is still an open question whether it is a moral obligation. What is needed to complete the proof is a moral justification of the institution.

The actual argument Searle offers, which is intended to serve as a counter-example to the thesis that an evaluative statement cannot be derived from a descriptive one, moves from the premise "Jones uttered the words 'I hereby promise to pay you, Smith, five dollars' " to the conclusion "Jones ought to pay Smith five dollars." An important step in the proof is the "other things are equal" premise that crops up twice. Now, other things being equal can mean at least two importantly different things. It could mean that there are not extenuating circumstances outside the scope of the institution that take priority over the obligation imposed by the institution. For example, when it is my turn to bat I am obligated to step up to the plate. But if I am told at that moment that my son has been in a serious car accident, my responsibility as a father to go to the hospital may override my obligation to go to bat. This is what Searle intends to avoid when he talks of other things being equal. But there is another sense in which it may apply.

The "other thing" could be a moral constraint that is necessarily violated by the following of the institutional rule. This is the point that the modified Russian roulette example was supposed to bring out. There don't need to be any extenuating circumstances for me to conclude that my real obligation in such a case is to say, "Let's stop." If the institution were a moral one then this problem would not come up. But Searle has not proved, nor indicated how one would prove, that a particular institution is a moral one. So, since what needs to be established involves an evaluative claim, regarding the moral status of the institution, in order to derive a moral ought from the rules which make up an institution, Searle has at least not derived a moral "ought" form an "is."

Nevertheless, has not Searle at least derived a certain kind of ought-statement from a certain kind of is-statement? Isn't that still quite a remarkable feat? How is it possible? Institutional facts, Searle explains in The Construction of Social Reality, are products of collective acceptance. For example the physical conditions and circumstances in which a person counts as a "batter," as well as the rule that a when a batter has three strikes he or she is out, and when a batter is out he or she has an obligation to leave the batting area, are all products of collective acceptance. So, what he has shown is that given collectively accepted facts, some of which are in the form of rules which involve placing obligations on people in certain circumstances, it follows that people who are in those circumstances count as being under an obligation. Thus, the bridge between the descriptive element and the evaluative element in the case of institutions is the collective acceptance of such a bridge, that is, the collective acceptance of a rule. As far as institutional obligations go, we can derive them only after we first collectively contrive them. Or, at least, that is all that Searle has shown.

References:
Cahn, Steven M., and Jeram G. Haber. Twentieth Century Ethical Theory. 1994.
Hume, Treatise of Human Nature.
MacIntyre, Alasdair. "Hume on 'is' and 'ought'."
Searle, John "How to Derive 'Ought' from 'is' " (in Cahn & Haber).
Searle, John. The Construction of Social Reality 1997.