[Exemplary Reasoning: Semantics, Pragmatics, and the Rational Force of Legal Argument by Analogy, 109 Harv. L. Rev. 925 (1996).]  Footnotes have been renumbered from the original.  The full text of this article is available.]

... This is the basic way in which reasoners deploy the structure of exemplary, disanalogical argument to distinguish cases that might, on first glance, seem relevantly similar.

Argument by disanalogy is the argument pattern exhibited in Mills v. Wyman,⁽¹⁾ a well known contracts case. At issue in Mills was the promise of a father to repay a "good Samaritan" for the latter's expenses in taking care of the father's deathly ill son.⁽²⁾ The father made the promise only after the do-gooder had rendered the services.⁽³⁾ The common law generally held that "past consideration is no consideration," and the issue in Mills was whether the father's promise was legally enforceable by virtue of "moral obligation" or moral consideration.⁽⁴⁾ The Supreme Judicial Court of Massachusetts, per Chief Judge Parker, decided that moral obligation alone could not provide the required consideration to enforce a promise.⁽⁵⁾ But the court was faced with some precedents in which judges had stated that moral consideration was sufficient consideration to support a promise. Reasoning by disanalogy, Chief Judge Parker distinguished those cases from Mills.

Capable lawyer that he was, Parker noticed that in all the cases in which it was said that moral obligation was sufficient consideration to support legal enforcement of a promise, there had been prior "valid" consideration for the promise that had been "extinguished by the operation of positive law."⁽⁶⁾ The leading example was that of a promise to repay a debt, which at some point had been legally enforceable (had sufficient consideration for its enforcement), but which later became extinguished by the operation of a discharge in bankruptcy. (Other examples included debts of infants and debts barred by statutes of limitations.) Some cases before Judge Parker said that the bankrupt's promise to repay the debt, made after the discharge in bankruptcy, was supported by sufficient consideration to enforce the promise. Those cases characterized the nature of the obligation on the bankrupt as moral consideration and made broad statements to the effect that "moral obligation is a sufficient consideration to support an express promise."⁽⁷⁾ According to Parker's interpretation of the case law, however, these authorities had spoken too broadly. On his reading, the law did not say that in all cases in which there was moral obligation (Parker made it quite clear that he believed that there was moral obligation in this case⁽⁸⁾, there was consideration to support a promise. To establish the narrower view, he constructed an argument by disanalogy that distinguished the prior cases from the one before him.

Recall that in the pattern of argument by disanalogy, there is a relevant dissimilarity between x and y that is sufficient to justify giving y a different legal treatment than x received.⁽⁹⁾ Here is a reconstruction of Parker's analysis by this pattern:
 

Target (y) = the case under consideration (involving reneged promise by father).
Source (x) = precedent cases, including those stating (wrongly, in the view of Parker, the "disanalogical" reasoner) that moral obligation was sufficient consideration for enforcement of a contract.
Shared characteristic: F: there was a morally obliging promise to pay that was not supported by additional new consideration.
Unshared characteristic: G: there had existed a prior legally binding promise that later became extinguished by the operation of positive law.
Inferred characteristic: H: there was adequate consideration to support enforcement of the promise.

Argument:
(1) x and y both have F;
(2) x has G;
(3) y does not have G (y has not-G);
(4) x also has H;
(5) DWR: any F is H unless it also has not-G (all things that are both F and G are H);⁽¹⁰⁾
(6) Therefore, the presence of F and H in x does not provide a sufficient basis for inferring the presence of H in y.

Several observations are due. Just as we distinguished the analogy-warranting rule from the analogy-warranting rationale, so too we will distinguish the disanalogy-warranting rule from the disanalogy-warranting rationale. Recall that rationales explain and justify rules. The disanalogy-warranting rule states the logical relation between the unshared characteristics (G in the example above - recall that the characteristics are shared in argument by analogy) and the inferred characteristics (H). In the Mills schema, step 5 articulates the disanalogy-warranting rule; this rule states that, logically speaking, from the presence of F in an item, one cannot infer the presence of H, but from the presence in an item of both F and G, one can infer the presence of H.

Typically, as in Mills, disanalogy-warranting rules impose additional conditions on the rules stated (or implied) in prior cases. That is, they rewrite the rule articulated by the earlier judge (the judge of the source case) by adding new conditions to the bundle of jointly sufficient conditions in the original rule. Thus, in Mills, apparently (and roughly speaking), the precedents Judge Parker was reading stated some rule like:
 

(i) Moral obligation is sufficient consideration to support enforcement of a promise.

(i) Moral obligation alone is insufficient to support enforcement of a promise, but moral obligation along with prior valid consideration that was extinguished by the operation of positive law are sufficient.

However unsatisfactory the court's actual performance, this case nicely illustrates the varieties of rational adjustment that a court might make - or might fail to make - when confirming or disconfirming analogy-warranting rules.
 

B. Is Abduction a Form of Mysticism?

I think not. Although abduction requires what is inevitably an imaginative and somewhat untamed moment of rational insight,⁽¹²⁾ it is nevertheless bound by significant rational constraints in settings calling for either inductive or deductive analogy-warranting rules. And in the latter setting, say, from the point of view of a legal interpreter who aspires to satisfy the rule of law norms discussed above, two very significant constraints - necessary conditions - guide her process of discovery in any plausible legal argument: the AWRs that are discovered must satisfy the entailment requirement, and they must sort particulars in a way that is plausibly explained and justified by AWRas. Again, because reasoners disagree, these conditions will not alone be thought sufficient to render reasoning by analogy justified. But underdetermination must not be conflated with a lack of rational constraint.