As Carl Becker, in his paper sharing my titular question, What Are Historical Facts? (Western Political Quarterly, Sept.1955--if you haven't read it, you must do so at once! If you don't have JSTOR access, feel free to hit up /r/Scholar, where I promise you'll be made right), observed, historians love to talk about facts. There is "the fact of the matter," "no getting around the fact," "the simple fact," "the foundation of fact," and of course, "the hard facts."
Most simply, an "historical fact" is a fact about the past. Something we know to be true about the past world. Except while most people would instinctively call this definition correct, there is a more interesting question--is it correct?
We're going to start with more first principles than Becker does, though his discussion will concern us next, but we are going to steal his example, which has since become the standard example (eg recently, Curthoys & Docker, Is History Fiction?). It is an historical fact that Caesar crossed the Rubicon in 49 BCE.
I see no reason to stray from Becker here because, when he wrote as now, it is a simple statement that almost everyone knows, and more importantly, knows to be true. No biography of history's greatest man will fail to mention it.
But is it a "fact" in the sense that it's a fact that gravity is proportionate to mass, for example? Well, no. Not even close. We are, in fact, several steps removed from that. Here's the actual fact: The copies of sources that survive tell us that it happened.
Yet our sources also tell us that a jocular Caesar assured his captors they would be crucified when he was abducted and held for ransom. This, however, is not an historical fact. What's different?
I point to this distinction not because I actually want to explore what's different, I trust it will be self-evident to most of my readership. Rather, it is to point out that "historical facts" do not exist as some objective, observer independent truths. They are arbitrated. The history plays judge and jury, and it is consensus as much--sometimes even more--as it is certainty that creates "facts." As Becker and I both note, the value of the Caesar example is not just that it is certain, it's that it is agreed upon.
It is important not to forget the role of the historian in creating historical facts. Sometimes the "facts" are unlikely to change. Augustus will almost certainly always have ruled Rome, and this will always be an historical fact. The evidence is just too good, too abundant, too decisive. The role of the arbiter is minimal.
But can we say the same about the "fact" (so Meier, Crossan, Fredriksen, Sanders, etc.) that Jesus died on a Roman cross around the year 30 CE (you knew I was coming to something like this!)? is that a "fact" in the sense Augustan rule of Rome is? Certainly not.
When recounting the "facts" of an historical narrative, or the "foundation of fact" upon which such a narrative is based, never forget that those facts were made, they do not simply exist.
Next time we'll move to Becker's discussion, and why there are simple statements, but no simple facts.