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Depending on who you ask, a “rule of thumb” is either a useful way of remembering an otherwise wearisome idea, or a phrase which harks back to an origin we might prefer to forget.
Some years ago, in the long-running “On Language” column in the New York Times, William Safire discussed this very topic (“On Language; Misrule of Thumb”, 25th January, 1998). He describes how, in George Washington University, an official had referred to a rule of thumb when discussing the university budget with a student newspaper. A student issued a response. She pointed out how “the phrase originated in English common law, where a man was permitted to beat his wife as long as the rod he used was no bigger than the width of his thumb”.
In 1782 an English judge, Sir Francis Buller, made a public comment to this effect and was duly subjected to the political caricaturists of the day. The best-surviving cartoon shows a judge laden down with sticks, with the accompanying script recounted by Safire as follows:
“[Judge Buller]: ‘Who wants a cure for a nasty wife? Here's a nice Family Amusement for Winter Evenings.’ Meanwhile a wife is shouting, ‘'Murder!'’ and a husband is shouting back: ‘Murder, hey? It’s Law you Bitch! It’s not bigger than my Thumb!'’ (available and transcribed by the British Museum here).
Judge Buller “excited general animadversion” following this “unguarded saying, which escaped him unpremeditatedly”, and promptly received the endearing nickname of “Judge Thumb” (William C. Townsend, The Lives of Twelve Eminent Judges of the Last and of the Present Century (Longman et al. 1846), Vol. I, p. 19). Safire goes on, however, to explain that the phrase was first printed much earlier, in 1692, with a meaning identical to that used today.
Yet the English common law origin has apparently been accepted in significant quarters. For example, in Davis v. Johnson  2 W.L.R. 182, Lord Denning M.R. stated at page 187:
“To go back for a few centuries, by the old common law a husband was allowed to beat his wife so long as he did it with a stick no bigger than his thumb. He was able, Blackstone says, to give his wife ‘moderate correction.’”
When you see the words rule of thumb, your mind may well turn to the escapades of “Judge Thumb”. For my part, I do think twice when I see the phrase used, as it has been in many judgments of O’Donnell J., who appears to have been casting a degree of doubt over the appropriateness of overuse of legal rules of thumb.
In Kilsaran Concrete v. Commission of Valuation  IESC 61, O’Donnell J. outlined the basis of these rules (paras. 34–35):
“[T]here is an inevitable tendency to reduce the law to a series of rules of thumb… Rules of thumb are very useful because they are understood to encapsulate in a simple form sometimes complex propositions which are both difficult and time consuming to establish from first principles. But it may however be necessary to check in any particular case, whether the rule of thumb precisely and accurately expresses all aspects of the underlying proposition.”
He expanded on this in DPP v. Colbert  IESC 69:
“6. It is the fate of even the most important judgments to be reduced to head notes then paraphrased in a few sentences in textbooks, later perhaps reduced to footnotes in texts, and finally pithy rules of thumb expressed by practitioners… However, almost every important general proposition can be pressed beyond its anticipated area of application with the possibility of error. While the student might yearn for a striking judgment enunciating the clear and straight line rule of general application, the reality of the law at appellate level is more often a process of ongoing adjustment with the statement of principle almost immediately followed by a qualification.”
Such an adjustment occurred recently in Merck Sharp and Dohme Corporation v. Clonmel Healthcare Limited  IESC 65, where the Court found it necessary to reassert the flexibility of the Campus Oil test which had, over time, become a calcified rule of thumb:
“30. Lawyers, whether judges, practitioners, teachers, or students, tend to favour propositions which can be reduced to some simple formulae that can be readily understood, remembered, and applied.”
But earlier in the judgment:
“27. … law develops incrementally and rarely proceeds in a straight line. Instead, there is an almost endless process of refinement, qualification, correction, and (sometimes) overcorrection. There is also a discernible tendency to reduce the approach taken in cases to rules which sometimes become calcified so it becomes necessary periodically to reassert the essential flexibility of the remedy…”
The fact that the law develops in difficult ways which may not lend themselves to simple rules was returned to in DPP v. C.C.  IESC 94:
“1. The incremental development of the law is not simply linear. Cases emerge with different facts which place principles previously announced in a different light, perhaps requiring reconsideration, qualification, and even, on occasion, a change of course. General principles announced by appellate courts are applied in a myriad of different situations by trial courts, and by repetition can become reduced to rules of thumb which over time may themselves require further clarification, adjustment or qualification. The development of the law in any field is a process of adjustment and correction.”
So, rules of thumb must be well vetted:
“35. Much of our daily life and our reasoning processes involve shortcuts, rules of thumb, statements of principle and generalisations… But it is necessary for a court to satisfy itself that they are well founded and applicable in every conceivable situation before it could be correct to decide a case merely on the invocation of generalised statements, however eminent the original source” (Balmer v. Minister for Justice  IESC 25).
Finally, we have examples of rules of thumb being used mistakenly by lawyers.
N.A.M.A. v. Commissioner for Environmental Information  IESC 51:
“43. In my view, this case at all of its stages manifests the difficulty of seeking to resolve difficult issues by reference to rules of thumb and proposed shortcuts…”
In Gorry v. Minister for Justice  IESC 55:
“66. … There is a lawyer’s tendency to attempt to reduce the law to clear-cut rules and shorthand phrases. In this field, one example is the development of the concept of the ‘marital family’ contrasted with the ‘non-marital family’. The position is, however, at least in my view, less clear-cut.”
In his piece, Safire includes a quotation from historian Henry Adams: “No one means all [they say], and yet very few say all they mean”. Perhaps it is the latter sentiment which has led to the Supreme Court’s thorough explanation (or gentle renunciation?) of such rules, at least in relation to difficult issues at the highest levels of the law. If that truly is the path being taken, and as alluded to twice above by O’Donnell J., it may be gloomy news for law students of the future.