A known unknown
Let me offer you a little puzzle. It’s about the famous land bill of Tiberius Gracchus in 133 BC. Here’s the evidence.
Livy writes (Periochae 58.1) that Gracchus’ land bill implied that no one was to own more than 1,000 iugera of public land. Then, there’s Plutarch, who lived a century later and states that no Roman was to own more than 500 iugera of land (Life of Tiberius Gracchus 8.2). There’s an obvious contradiction here, but things are even more complex than that. Some two generations after Plutarch, Appian of Alexandria informs us that Gracchus’ land bill provided
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 (Civil Wars 1.8).
What to make of this? 1000 iugera, 500 iugera, or 500 iugera plus 250 iugera for every son? Under normal circumstances, we would try to check the sources of our sources. Was this information from, say, Valerias Antias (who was not above inventing events) or from a more reliable author? Unfortunately, Plutarch and Appian are not in the habit of mentioning earlier authors, while Livy’s History of Rome since the Foundation of the City, in which sources were acknowledged, has been handed down to us by an epitomator who believed this to be irrelevant. So, we cannot proceed by checking the origin of the information.
There’s a different approach: we usually have a general idea about the reliability of an author. Appian, of course, ranks among the very best historians in Antiquity, while Plutarch is in the first place a moralist, who is less interested in history itself than in the lessons we can learn from the actors. The fact that he talks about “land” while the debate is about “public land”, proves that he’s not really interested in getting the facts right. Livy’s history becomes increasingly reliable when he comes closer to his own age, but the epitomator may have left out important information. In short, we would prefer Appian, but he is also the youngest of the three authors, which is usually not a good sign.
So, we’re left with a little puzzle. French historian Jerôme Carcopino thought he knew the solution: 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, and it has been repeated by several other historians, but this theory is also contradicted by all available sources.
And that brings us to a bigger puzzle. In this case, we had three contradictory sources. We realized there was a problem, started to ask questions, and made some guesses. This was, to use an expression made famous by Donald Rumsfeld, “a known unknown”. But what if we had had only one source of information? We would not have recognized the unknowns and I am afraid that we would have copied the information without asking further questions.
One source of information means, essentially, that we have no information at all. This is the principle called testis unus testis nullus, “one witness is no witness”. In my personal view, the study of ancient history is valuable because it trains us in thinking about the known and unknown unknowns that are inherent in all human understanding.