In the roleplaying game Legend of the Five Rings, set in a fantasy world heavily based on historical Japan, there’s a lineage known as the Kitsuki who are notorious for training a very odd type of investigator: instead of relying primarily on testimony, they look at physical evidence.
How exactly this description gets interpreted has varied throughout the years and editions of L5R, not to mention the way individual players or writers of fiction for the game line have chosen to spin it. Sometimes it gets taken to an extreme where literally the only thing magistrates care about is what people are willing to stand up and swear to, and whatever is said by the highest-ranking person is deemed to be The Truth, the end. At other times, it’s more nuanced; it isn’t that other people never consider physical evidence, but rather that the Kitsuki are Sherlock Holmes, pulling together insignificant-seeming details and abstruse bits of information to assemble a picture nobody else can quite see.
“What counts as evidence” is a contentious question, and whose answer has changed not just in the distant past, but quite recently. Anybody who’s watched an episode of CSI or some other modern-set police procedural has seen the array of sophisticated forensic techniques we’re capable of deploying nowadays — though the actual depiction of said techniques is often howlingly wrong. Which has become an active problem in trials, when jurors expect forensics to work like they do on TV. But that in itself is a radical shift from the days of the O.J. Simpson trial, when DNA evidence was a relatively new concept, and jurors were purportedly so adrift in a sea of technical jargon that it was easy for them to accept the defense’s argument about its unreliability.
But forensic science itself is not new. The earliest book on the subject comes from thirteenth-century China, a text variously called Collected Cases of Injustice Rectified or Washing Away of Wrongs, by Song Ci. Early investigators had to tackle questions like how to identify badly-damaged bodies, estimate time of death (which varies based on factors like weather conditions), detect injuries not visible to the naked eye, spot the use of poison, and more. Chinese officials also began taking fingerprints (or hand-prints, or foot-prints) as early as the ninth century, and the practice of fingerprinting criminals may even go back all the way to Hammurabi in the second millennium! It’s unlikely that early uses recognized the uniqueness of fingerprints — much less was collecting latent prints from objects at a crime scene — but they were still seen as identifying marks, just as facial features, scars, moles, and birthmarks were. They helped to identify a given person as a known criminal.
Other kinds of evidence are no longer considered valid in modern Western society. During the Salem witch trials in the late seventeenth century, “spectral evidence” was considered valid: if a victim testified that they had a dream or vision of an alleged witch tormenting them, that was evidence of witchcraft. (Even at the time, some people had doubts — not because they questioned the veracity of the testimony itself, but because the Devil could take the shape of an innocent person to incriminate them.) Cruentation was the medieval belief that a murder victim’s body would spontaneously bleed or otherwise show a sign when touched by the killer; refuting that one required a better understanding of post-mortem science, to explain why a corpse might begin to leak on its own. And compurgation was the practice of swearing to your own innocence and then getting some number of people, often twelve, to say they believed you — which rested on a framework of belief surrounding oaths that will have to wait for a future essay.
There’s also the question of how the evidence is obtained and processed. In the United States, this is referred to by the rather poetic metaphor of “fruit of the poisonous tree,” meaning that if the source (the tree) is tainted, than so will anything taken from that source (the fruit). Generally speaking, if evidence has been obtained illegally, it is not admissible in court — though, as with everything legal, there are loopholes and exceptions.
This is why modern practice in many countries requires protective measures like warrants before authorities can break in somewhere to gather evidence. All those amateur sleuths in fiction who break in somewhere and find proof that the bad guy is doing bad things? Their evidence won’t stand up for a moment in court (because it won’t be allowed there in the first place). And as forensic science becomes more important in our trials, we’re having to take stringent precautions to avoid contaminating the evidence with foreign DNA or fibers or chemicals that could affect the results.
Mind you, we aren’t so stringent everywhere. There’s abundant evidence showing that eyewitness testimony is profoundly unreliable . . . yet our default impulse is to trust it. People’s memories are faulty; they’re easily swayed by bias (e.g. testifying that they saw a black man commit a crime, when the perpetrator was white); it’s upsettingly easy to get someone to “remember” a detail they never saw, without even meaning to do so. But we’re wired to trust our own memories — if we didn’t, we’d collapse in a puddle of doubt — and we’re social creatures, so hearing someone swear “I saw this happen” is extremely persuasive.
Even confessions aren’t wholly reliable. Nowadays we consider a confession obtained via torture, or even just via other forms of coercion, to be inadmissible, because people will swear to all kinds of things that aren’t true just to make the pain stop or to avoid a threat. But that’s a modern attitude: in ancient Rome, by contrast, a slave’s testimony wasn’t valid unless they were tortured first. Slaves were considered to be of such low moral character that they would readily lie; the truth had be forced out of them.
Like oaths, torture is a topic for another (much grimmer) day. But there’s rich worldbuilding potential in the topic of evidence, especially for anybody who aspires to write a mystery plot in a fictional world. What kinds of evidence might futuristic technology be able to extract? Is the testimony of a ghost considered valid? What kinds of forensics are possible within the tech level and scientific knowledge of your setting, and how willing are the authorities to accept proof that isn’t immediately visible or comprehensible to the layman? The conviction or acquittal of your character may rest on the answers to these questions.