§25. The Evidence of Law

Practically everything I discuss in this chapter happened in a university or was done by university-trained intellectuals. That has not been the case to this point, so I shall begin with some notice of these uniquely European institutions.

The first university in Europe was founded in the north Italian city of Bologna in 1088 for the purpose of studying Emperor Justinian’s Corpus Juris Civilis, the largest part of which (the Digest) was newly recovered in 1070. Modern law, English and Continental, descends in an unbroken tradition from the work of these glossators. Medical faculties soon followed, and nat­ural philosophy flourished in association with learned medicine, especially in north Italy. Law made its own contribution, however, not just to science but specifically to empiricism, with principles for reasoning about evidence and components of proofs that fall short of apodictic demonstration.⁸

The first science in Europe was a legal science, from the twelfth century, and a serious argument can be made that these first jurists were progenitors of modern science. They were empirical. “They were the first Western scholars to see and develop not only empirical tests of the validity of ge­neral principles but also empirical uses for such principles. That the empir­ical data they examined were existing laws, customs, and decisions did not make their achievement any less astonishing.” And they were experimental. “Experience, including the experience of applying rules in concrete cases, was viewed as a process of constant feedback concerning the validity both of the rules and of the general principles and concepts that were thought to underlie them”⁹

The use of evidence in legal thinking drives the development of methods for reasoning under uncertainty.

Medicine made experience matter, and invented the indicative sign, the visible indication of a hidden cause, but was less inventive in dealing with complex and even contradictory evidence. It was from jurisprudence that Europeans got their first experience in judging a balance of probabilities. In European law before the twelfth century, methods of proof were oaths, battle, and ordeal, all equally valid. But by the twelfth century doubts began to multiply and confidence in these methods melted. Legalists developed alternatives, which became the law of evidence. Where law is in doubt and certainty impossible, different opinions have to be reckoned with, which requires principles of evidence. The parties can't just fight it out. The or­deal will be one of the logos and before peers, not an inscrutable judgment of God.¹¹

One of Europe's most original legal concepts is the half- proof (semiplena probatio). This recognizes a middle ground between full proof and no proof. It also recognizes that several half-proofs can be combined, and that full proof can even be cobbled together in this technical way. Having dis­covered half-proofs between full proof and no proof, it seems inevitable that lawyers would discover something halfway between a half-proof and no proof, which they called indications (indicia).

By the fourteenth century law’s style of qualitative reasoning about probability had spread to a variety of contexts in which people evaluate the worth of evidence. Greek doctors were divided on the admissibility of hypotheses largely because they had only a vague idea of how to evaluate evidence for and against them. It was from the experience of legal rea­soning about evidence, a Roman legacy, that Europeans learned their first lessons on statistical significance. If, on a hypothesis, a given result would be unlikely, then obtaining that result tells against the hypothesis, while the verification of a nontrivial consequence renders a hypothesis more likely. Mathematician George Polya described this as “the fundamental in­ductive pattern,” and it was in reasoning about legal proof and evidence that Europeans discovered it.¹³

Law has this distinction from medicine: it has to apportion causes. A physician may have no idea what caused a disease, yet be a good healer. The Empiricists made this a point of honor. When doctors disagreed, it was usually not on diagnosis, prognosis, or therapy in a given case; more typical disagreements arose from theoretical disputes about the hypo­thetical cause of disease. These are disputes that good medical practice does not have to resolve, whereas judicial decisions must confront a bat­tery of evidence, none mathematically conclusive, and make a probable determination. Ancient medicine was not conducted in such a setting, and provided little practice in thinking about how to weigh empirical evi­dence. Almost everything in ancient medicine is an all-things-considered judgment that expresses the physician’s art, not a professional analysis of evidence. Medical diagnosis is a skill, a techne, and operates, as techne typ­ically does, without explicit procedures. At its best it is not probable but sagacious.