The standard disclaimer for my fencing-philosophy posts applies. That is to say, you are likely to disagree with this post, but I have found these things to be Good and True thus far. They are here for you to peruse and either accept or reject, as you wish.
In all competitive games, there are "good" moves and there are "bad" moves. These change depending on positioning, for any interesting game. In SCA fencing, a good example of a "bad" move is remaining still when your opponent is thrusting into your face. Similarly, a "good" move would be anything that prevents you from being stabbed, while the "best" move would be to do something that causes you to live, but also causes your opponent to be stabbed. A single-tempo parry-riposte is an example of such a movement. But let's pause there for a moment.
There is a concept of "bad", "good", and "best" moves. However, if your opponent knows what you are going to do, they can frequently thwart your move, even if it's the "best" move. This leads to a game of anticipation, which we will return to in a moment.
The Problem Space
What defines the "best" move? In general, it is the maneuver that has the highest chance of success, if your opponent is solely reacting to the current situation, rather than trying to predict your next move. "Success" is intentionally ill-defined here. For me, I value not dying over killing my opponent in most instances. Sometimes you want to maximize the chance that you will kill your opponent, even if that will result in your death. So, the "best" move is the one with the highest "success" rate in the problem space.
What is the problem space? The problem space simply put, is the list of all actions that your opponent could take. For the China MiƩville fans out there, imagine the Possible Sword from The Scar. Imagine your opponent's sword, arm, and body blossoming outwards in all of the directions that they could possibly move. That is the problem space.
The "best" move is the one which succeeds in as many of those possibilities as possible. A "perfect" move is one that thwarts literally all of those possibilities and forces your opponent to lose, regardless of the actions that they perform. This obviously excludes actions that are not in the rules of the game, like yelling out "HOLD" when you are about to die and such.
At this point it should be obvious why constraint is very important in fencing. If you can restrict the size of the problem space, it becomes much easier to react to the full problem space rather than having to just make a guess and go. This is why I personally love the outside line in Italian single rapier - it is a very restricted problem space, in which it is relatively clear to see when and how you can attack without dying.
The True Game
This is a concept I have mentioned before in practice reports, but I have never really defined. The "true" game, in any competitive game, is one in which you progressively try to restrict your opponent's options in different ways until you can take a "perfect" move and win the game. Similarly, your opponent will try the same thing. This requires a full knowledge of everything you can do with a sword. We can all aspire to playing the "true" game, but in our version of fencing, the problem space is wide enough that this is perhaps impossible. People can thwart your attempts to do this by using non-standard equipment or by assuming non-standard postures. We will get back to that in a moment.
The Anticipation Game
As I said above, at a certain point you will learn what the "best" response for your opponent is. Or at the very least, you will learn what they think the "best" response is. You then may decide to short-cut the issue and react to what you think they will do. And this will probably work, if your opponent is not a master. Similarly, your opponent may learn to try to trick you into reacting a certain way, and then respond to the reaction that they predict.
This is a degenerate game. I used to play this almost exclusively, and I play it when I get lazy. But it is degenerate and bad. You will hit a wall and be unable to climb it, if you try to play this.
Reacting to what you think your opponent will do might gain you success in the short-term. This game quickly gains feedback and static from how you and your opponent dance together. The things that you learn are "good" might just be cognitive hiccups in your opponent. Maybe that thing which you think is "bad" is just the thing your opponent expects you to do.
If you find your opponent playing an anticipation game, you need to learn to be a rock. You should learn to draw out your opponent's responses, and only respond after they actually move. You should learn the full space of things that your opponent could do, and then react in ways that restrict their options. This is a frustrating way to play, but it will make you lose less. This is different from winning more, and vastly preferable.
If you find yourself playing an anticipation game, you should stop. Fixing this requires you to think on what you do and what you react to. It requires you to figure out what the actual "good" and "perfect" responses are in your game. This is the hard mental work which lets you get better. If you are a historical practitioner, you should review your chosen master or masters. If you are carving your own path, you need to think harder.
Remember the thought-exercise of your opponent's arm blossoming out into all possibilities, and think about what you can do to restrict their movement. If you can't find a way to safely restrict your opponent's options, then you need to either find a new technique or you need to avoid that situation entirely. Random chance is anathema to victory.
The One-Off-From-True Anticipation Game
This bears special mention. If you understand the "true" game very well, then your Anticipation Game can be very, very good. You may beat much stronger fencers than you by anticipating their actions. It is debatable whether anticipating the best move of the "true" game is the actual best move. Giganti is in favor of this, saying that creating false openings so that you can anticipate your opponent's actions is a good idea, in plates 30, 31, and 32. I personally disagree with this, because if your opponent realizes your intent, they can foil your false opening. At that point it becomes a simple game of rock-paper-scissors rather than chess.
This also relies on your opponent fighting in a way that is inside of your local meta-game.
The Local Meta-Game
It is nigh-impossible to know what the shape of the "true" game is. This is the art to which we all aspire, but it will forever remain out of reach. When we get close to the "true" game, we form our own separate version of the "true" game, in which we use some and don't know their counters. To us, those maneuvers are "perfect".
But then Johnny Midrealm waltzes in and fences in an entirely different way. Nobody understands his game. Maybe he does well, maybe he does badly because he doesn't understand us either. Things get shaken up. Some things which we thought were good turn out to be terrible. The opposite is also true. Even if Johnny was trying to fence the "true" game as far as he knew it, it is very possible that these people fight entirely different ways. Both parties gain quite a bit from the experience, and both parties gain a better knowledge of the actual True Game.
The situation of being the only guy at a particular practice which does a particular thing is similar. The only lefty will know the left-versus-right fight better than the righties do, and righties will learn quite a bit from fighting them. The only longsword-user will know how to longsword super well, and have an advantage from that.
This is why both manuals and fighting people from other areas are the best. Those two things are the quickest way to shake up your internal understanding of the "true" game, and bring it closer to the actual True Game.
The True Game, Redux
The True Game is what you should be fighting, whatever that is. There are many paths to the True Game, and the shape of it is different depending on your opponent, but you should learn to fight in ways that are straightforward and right.
Perhaps some deceit is good as well, but if you fight the True Game then you will never have those moments of "aww, he anticipated my trick" because you never use "tricks". You learn from "tricks", and then you find the places that your "trick" is an unstoppable, crushing wall that cannot be escaped or circumvented. You learn principles and you learn to apply them on the fly to new situations, so that you are never surprised. You learn to adapt to the situation and strike true such that not only do you win, but you make it impossible for your opponent to win. You surgically remove chance and gambling until only victory remains.
You play chess, rather than rock-paper-scissors.