The importance of written sources
Some time ago I wrote an article called Questions on What Is, and What Is Not HEMA, and recently I’ve been thinking about that question a little more. Firstly, I think that a differentiation must be made between historical European martial arts, as in martial arts that were practiced historically in Europe, and HEMA, as in the modern sporting practice. Viking sword and shield, as in the styles of fighting done with a sword and shield by the Scandinavian cultures that we refer to as the Vikings today, was clearly a historical European martial art; it was a martial art used historically in Europe. However, I would also argue that it is not part of HEMA, the modern discipline.
The reason for this distinction is sources. A Norse warrior living in the 9thor 10th centuries had no need to examine written sources to see if the way he was fighting with a sword and shield was historically authentic, whereas a 21st century practitioner cannot just fight with a sword and shield and claim his method of fighting is historically authentic. They must, or at least should, use evidence to back up their claims and demonstrate that what they are doing is likely to be historically authentic.
Anyone can fight with a system for which we do not have evidence, but they cannot know if what they are doing is in fact a historical European martial art or not. The only way to tell if what you are doing is in fact historically authentic or not is to study sources. If you do not study sources, then the fighting styles you practice or teach may or may not be historically authentic, although I would go so far as to say that a modern individual living in a modern context is unlikely to be able to re-create a historic system accurately without clear guidance. I would go further and say that not only must a HEMAist study sources; there are only some sources that can be studied fruitfully with the end goal being to recreate a system that is likely to be historically authentic.
Written sources are generally far superior to other kinds of sources in that they can convey information a lot more clearly, and can generally convey a greater amount of information as well. For example, if you look at my last article The Problem with the Broadsword and Targe Sources, you can see a brief overview of the differences between those sources. The Penicuik sketches are a series of drawings with no text, while Pages’ treatise contains clear written instructions on how to stand in guard, and what techniques to carry out from those guards. We could perhaps supplement our knowledge of how to fight in a Penicuik sketches inspired manner with other written sources describing how the Highlanders fought, either in one on one duels or in battles; however the end result still has less chance of being historically accurate, compared to following a written treatise with explicit instructions on how to fight.
We can say with a relatively high degree of accuracy how to fight in a historically authentic manner within the Liechtenauer tradition of longsword because we have many textual sources describing not only the guards and techniques we should use, but more importantly they tell us how we should approach a fight. For example, I can say with certainty that trying to gain the Vorschlag, or using Indes and Fuhlen, or using the Five Strikes are all important elements of how you approach a fight within the context of Liechtenauer’s longsword system. I can say why some approaches to the fight are correct within the framework of Liechtenauer’s longsword system, and why others are incorrect.
To return to the example of Viking sword and shield, it would impossible to create a system for sword and shield that would be as historically authentic as a recreation of Liechtenauer’s longsword. It would be possible to create a system that is plausible, that is informed by what little evidence there is, that seems sensible, and that could have been used. Without clear, descriptive, written sources though, I do not see how such a system could reach the same standard as we have for a system like Liechtenauer’s longsword. I could not say anything with certainty about how one should approach a fight, nor could I say with certainty that what I am doing is likely to be historically correct.
It is often said that HEMA is a broad church, and that is open to a wide variety of approaches. This may be a controversial opinion, but I believe that some approaches, and some focuses, are more likely to produce valid results than others. If an approach is not likely to meet a certain standard in terms of how likely it is to produce a historically authentic result, then I do not believe it really has a place in HEMA.
Finally, I want to quickly address the idea of experimental archaeology. I sometimes hear people refer to experimental archaeology, and taking inspiration from that to justify hypothesising systems with a lack of written evidence. I have never met a trained experimental archaeologist, as far as I’m aware, and I would be dubious at any attempt at claiming to use experimental archaeology without input from a trained experimental archaeologist.
My current point of view is that any study that does not principally revolve around written treatises is not really HEMA, given the lack of guarantee that the end result will be likely to be somewhat historically accurate. I’m open to having my mind changed about that, if I saw a systematic explanation for how something like experimental archaeology can be used to study system for which no or little written evidence exists, and how a reasonable degree of historical authenticity could be assured.
Until then, my point of view remains that HEMA is the study and application of written historical European martial art treatises.
 We can of course never prove that what we are doing is 100% authentic, and the different contexts in which we live and train mean that practicing in a way that is 100% historically authentic is impossible.