Testis unus testis nullus ("one witness is no witness"): name of a problem that is created when historians have only one source - they cannot control the information and are forced to accept it.
Historians have a lot in common with astronomers looking at, for example, the Pole Star: they can not observe the object of their study directly. Astronomers can only look at light emitted four centuries ago, and historians have access only to written sources and archaeological remains. Observing historical facts is as impossible as directly observing the Pole Star. We can only study our object's fall-out, consequences, produce. This means that historians can never be real scientists, who can check and recheck the facts. Caesar was murdered only once, and we have only a handful of written sources as evidence.
However, there are degrees of certainty. Let's take a look at the land bill proposed by Tiberius Gracchus. There are only three pieces of evidence:
- Livy: "Tiberius Sempronius Gracchus carried a land bill. No one was to own more than 1,000 iugera of public land."note[Livy, Periochae 58.1.]
- Plutarch: "A law was enacted forbidding the holding by one person of more than 500 iugera of land."note[Plutarch, Life of Tiberius Gracchus 8.2.]
- Appian: "He brought forward the law, providing that nobody should hold more than the 500 iugera of the public domain, and added a provision that the sons of the occupiers might each hold one-half of that amount."note[Appian, Civil Wars 1.8.]
So, who are we going to believe? Livy's 1000 iugera of public land, Plutarch's 500 iugera of land (unspecified), or Appian's 500 iugera of public land plus 250 for every son? Many historians harmonize these bits of information, saying that the land bill allowed a man to own 500 iugera of public land, plus 250 for his two first sons. This may well be true, but it is also contradicted by all sources.
At first sight, the situation is desperate: there is a problem, and there is no way to solve it. On the other hand, we can at least discuss which source we prefer. Appian, who understood how the legal process worked? Plutarch, who often had access to reliable sources? Livy, a native speaker of the language in which his sources had been written? We can at least make an educated guess, because we recognize the problem.
Now compare this to the situation in which we have only one source - for example, Cassius Dio's remark that Caracalla awarded the Roman citizenship to every freeborn male in the Roman Empire "to increase his revenues".note[Dio, Roman History 78.9] We have no choice but to accept it, because we do not recognize that there may be a problem. Similar examples:
- Only one author (Herodotus) states that Cyrus the Great conquered Lydia before Babylonia.
- Only the Ptolemy III Chronicle states that Ptolemy III captured Babylon.
- Only Matthew writes that the Jews in Jerusalem shouted that Jesus' blood would be on them and on their children.
The pieces of information mentioned in this little list are less "hard" than the land bill of Tiberius Gracchus, which is at least mentioned in several, albeit conflicting sources. We can realize that there is a problem. However, if we have only one source, we cannot even recognize which problems there might be. Ever since the days of the Vienna Circle, philosophers have called statements based on uncontrollable information, "meaningless"; you may also use the old Latin proverb testis unus testis nullus, "one witness is no witness".
Does this mean that we must reject all information based on one single source? That would be exaggerated, if only because we have so preciously few sources. Much depends on the type of source. The Ptolemy III Chronicle is almost contemporary with the events it describes, and belongs to a series of texts that can often been verified. Another criterion may be plausibility: Herodotus' statement that Cyrus conquered faraway Lydia before he captured nearby Babylon is a bit odd. On the other hand, we can not reject every bit of information that strikes us as implausible - if we think we already know what happened, we may as well abandon historical investigation.
This article ends without solution: the problem exists, but there are no easy rules to deal with it. Yet, historians are well-advised to keep in mind that, while there are known unknowns (like the precise phrasing of Gracchus' land bill), there are also many unknown unknowns. Probably more, but how can you know?